The feminist grandee with her eyes firmly fixed on Number 10

When Jeremy Heywood, Permanent Secretary at 10 Downing Street, told his staff Harriet Harman was about to move in as Gordon Brown's holiday stand-in there was a groan. Heywood, a popular figure who used to run Tony Blair's private office, raised a weary hand. "I know, guys," he said. "But please don't shoot the messenger."

Such was the mood that greeted Ms Harman this week, as she became the first woman since Margaret Thatcher to occupy Britain's driving seat. It could have been a moment for rejoicing, especially among those who feel Ms Harman is the best choice to replace Gordon Brown in the run-up to an apocalyptic general election. Yet the sourpusses of Downing Street refused to enter into the epoch-making spirit of it all.

First, there were dark mutterings when Ms Harman's aides made it clear that at 10.30pm each evening she would require a briefing on the following day's headlines and news stories. Then there were heavy sighs when Ms Harman's people ordered a daily 7.30am conference call and a meeting in the Cabinet room (Ms Harman in the chair) at 9am sharp.

As her demands blew an unseasonal chill through Downing Street, the prime minister's official spokesman could hardly bring himself to acknowledge her significance. Mr Brown, he said unequivocally, was in charge. But, as everyone knew, he was 120 miles away in Suffolk, ostensibly enjoying a family holiday.

So what was Ms Harman doing in his chair, if she wasn't running the country? The spokesman would only concede, a touch grudgingly, that she had held meetings in Downing Street. He even thought better of this and said "she has been in meetings".

The spoilsports could not stop her using her Downing Street moment to create a little history. Ms Harman, Minister for Women, today announced dramatic new changes to the homicide laws, mostly by introducing a femicide law. No longer will husbands be able to say they were provoked into killing nagging wives. Spouses who fear serious domestic violence, however, can claim they were forced to kill. Ms Harman, arguably the most ardent feminist of her political generation, has been pushing for these changes for years. To be able to launch them from 10 Downing Street must count as a career peak.

Yet, at the very moment of her triumph, with the levers of power tantalisingly close, she was denied the wholehearted endorsement she may have felt was her due by the po-faced minions who seem to resent her. Hadn't they read The Times? The paper's commentator William Rees-Mogg argued yesterday that Ms Harman could be Britain's Hillary Clinton. For Labour to change "the climate of debate", he wrote, Ms Harman would be better than the men among the front-runners in any future leadership contest.

Since she currently trails the favourites - David Miliband, Jack Straw, Ed Balls, etc - it might be thought Lord Rees-Mogg was being deliberately controversial. But her presence in Downing Street today derives from the fact that she has already proved a considerable political operator.

When John Prescott stood down as deputy party leader and deputy prime minister following Tony Blair's decision to leave Downing Street there was a rush for the number two job. Peter Hain, then work and pensions secretary, launched a bid, as did Alan Johnson, then Education Secretary, now Health Secretary. They were joined by a handful of also-rans. Many would have placed Ms Harman in this category but she borrowed £10,000 from her bank, extended her mortgage by £40,000 and fought a tigerish campaign.

Later it emerged she had also accepted £5,000 from Labour's mysterious benefactor David Abrahams through a third party, which is against the rules. She gave the money back and survived the episode without apparent lasting damage.

In the initial round of voting she emerged the first choice of Labour Party members. She was also popular with Labour MPs but the unions snubbed her. As the intensely complex series of votes unfolded and outsiders fell by the wayside it seemed Alan Johnson might get it but she pipped him in the fifth round by less than 1 per cent of the poll.

She was duly made deputy leader of the party but not deputy prime minister. MrBrown chose not to follow Mr Blair's example and pointedly declined to create a deputy position. Instead, she became party chair, leader of the House of Commons and Secretary of State for Equalities, enough to keep most people out of mischief.

The deputy PM role might have eluded her but her victory in the deputy leadership election made it hard for Mr Brown to pass her over when it came to filling the vacation vacuum at Number 10. Chancellor Alistair Darling and Justice Secretary Jack Straw will also have a turn during his three-week absence. It might be thought the staff in Downing Street would recognise impending greatness and knuckle down to a Harman regime, even for a week. But for reasons that apparently perplex even her, Ms Harman is not universally admired. Some would go so far as to say she is among the most disliked people in Westminster politics. At first sight, this may seem strange because she has a terrific record for commitment and hard work. Hard work was a creed in Ms Harman's family. Her father was a distinguished Harley Street physician and her mother, Anna, was a qualified lawyer who once entertained parliamentary ambitions of her own. Ms Harman is a niece of Lady Longford and the author Lady Antonia Pinter is a cousin. The family has a tradition of high-achieving women. Her paternal grandmother was one of the first women doctors admitted to the British Medical Association.

Ms Harman's mother encouraged all four of her daughters in their careers: Harriet and her sister Sarah followed her into law. Harriet attended St Paul's School where one of her former teachers described her as "not at all academic, not a jot remarkable and a bit dull". Others remembered an aggressive child who often pushed and shoved other girls in the playground.

She secured a place at York University and took a middling degree in politics, before turning to law and qualifying as a solicitor. This, she said later, was the turning point in her life. In 1974, aged 24, she was hired as a solicitor at Brent Law Centre. Some people viewed this entirely respectable operation as a hotbed of revolutionism. It specialised in helping workers fight for union representation and became involved in the notorious Grunwick dispute. In the long, hot summer of 1976 workers picketed the Grunwick film processing factory in Willesden after accusing the management of exploitation. The company responded by bussing in workers, a tactic that led to violent clashes. In the thick of it was young union official Jack Dromey. He had been campaigning for the factory workers' right to union representation and became, for many, the hero of the hour.

Dromey was a firebrand typical of the age. He came out of a crowded home in Kilburn where his Irish father was a road digger. His background could hardly have been more different from that of the young woman from Brent Law Centre who took up the Grunwick workers' case. But when Jack Dromey and Harriet Harman met on the Grunwick battlefield something clicked that appears to endure to this day.

Dromey rose through the trade union movement and is now treasurer to the Labour Party. Over the years, he acquired a certain passivity and has been known to call himself "Mr Harman". They have three children, two boys and a girl, all now grown up.

Dromey once said, rather disarmingly, that he never expected to have a daughter who would play the piano.

He probably never thought he would have demonstrators on his roof, either. But protesters from Fathers 4 Justice have made the Harman home in Herne Hill a target. They seem to feel Ms Harman has a down on men.

In 1982, she was elected MP for Peckham. It is currently thought to be Labour's 15th safest constituency and Westminster wags have noted that if the voting pattern in last week's Glasgow East by-election was repeated at a general election she would be among only a handful of Cabinet members who would hold on to their seat.

When Ms Harman entered the Commons (she was seven months pregnant at the time) Labour had only 10 women MPs. The new member for Peckham swiftly made it her business to change that. She set up a parliamentary women's group and was in the forefront of the movement to introduce womenonly shortlists. The then labour leader Neil Kinnock promoted to her to his front bench and she worked closely with MrBrown in the early 1990s. When New Labour won its election landslide in 1997 she was appointed secretary of state for social services. She was also made minister for women.

In 1990 she co-authored a report, The Family Way, which questioned the value of the nuclear family and appeared to disdain the traditional role of a man as pater familias. Ms Harman's unblinking commitment to feminine equality and political zealotry made a number of enemies among senior civil servants. One recalled that when he held a door open for her she rounded on him, accusing him of patronising sexism.

SHE acquired the sobriquet Harriet Harperson and a story began to circulate that her children used her surname, not that of their father. This appears in some internet sources as fact but it is not true.

Perhaps it's a measure of her unpopularity that such stories exist. When she by-passed her local comprehensive and sent her second son Joe to a selective grammar school 10 miles from the family home she was branded a hypocrite. Her elder son Harry was already attending the Oratory school. The Tories turned up the heat over the issue, but Blair came to her rescue. His own son, Euan, was Harry's contemporary at the Oratory.

For some time there was a sense that while Ms Harman was a capable minister, her steamrolling feminism, general lack of charm and a speaking clock manner of delivery would tend to keep her away from serious power. But there is evidence of a late flowering.

Against expectations, she was allowed to stand in for Mr Brown at Prime Minister's questions during the G8 summit earlier this month. Opposite her was William Hague, one of the best performers in the house. Astonishingly, Ms Harman seemed to get the better of him. He taunted her about her prime ministerial ambitions and Mr Brown's new policy on food. Why, snapped Ms Harman, would Mr Brown care to take diet advice from a man who drank 18 pints a day? Mr Hague politely wished her well in her campaign to get rid of Mr Brown.

Of course, there is no such campaign. She speaks lightly of her spell behind the famous black front door and says she is merely "minding the shop". It's her 58th birthday tomorrow and who could blame her if she has a little celebration in the house that has shaped Britain's history? The Downing Street regulars might huff and puff about bossy interlopers, but even they wouldn't begrudge her that, would they?