Sorry, Gordon, but you are in for a long wait

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Tony Blair hosted a Jubilee dinner for the Queen at Number 10 last night with the four living past Prime Ministers and relatives of the five deceased premiers of her reign. I don't know what she made of this tableau of the remarkable, the mediocre and the awful. I do know that a glance around the table will have been a source of satisfaction to Tony Blair. Clocking up five years at Downing Street this week, he is already poised to outlast virtually all of Number 10's other tenants of the past 50 years.

He has already and easily beaten Eden, Douglas-Home, Heath and Callaghan. Only in the ranks of Gordon Brown's most fanatical admirers is it doubted that Mr Blair can survive another 18 months. That will see him surpassing Attlee and Macmillan. Reaching the next election will leave behind Wilson and Major. Only among Iain Duncan Smith's most misty-eyed followers is it considered likely that Mr Blair could lose the next contest for Downing Street. A triple whammy of victories will match the feat of Margaret Thatcher.

A prolonged period at the top is a condition of becoming an important Prime Minister, but it is no guarantee of being an outstanding occupant of the office. One of his aides frames the crucial question in this rather morbid fashion: "If TB were shot dead tomorrow, what would people say? Once you've blown away all the froth, they'd say this is one of the most successful governments there's ever been." Mr Blair has passed the basic competency test of being Prime Minister by not inflicting a catastrophe on his country. He teetered on the brink of disaster when the fuel protests exploded in the autumn of 2000 and the Millennium Dome was a dreadful joke, but he has not been responsible for a humiliating debacle like Eden's Suez, Heath's Three Day Week, Callaghan's Winter of Discontent or Major's Black Wednesday.

His government has got the basics right by delivering more jobs and sustained growth with low inflation and interest rates. Power has been irreversibly devolved to Scotland and Wales, a historic shift achieved in spite of his own centralising impulses. No other Prime Minister represented around the table last night has got so close to establishing an enduring settlement in Northern Ireland. Though the man himself is something of a social conservative, his government has tentatively pushed forward the progressive agenda with the equalisation of the age of consent.

Some of the landmarks have been unanticipated, not least by himself. During the most perilous moment of the Kosovo war, Mr Blair ruefully remarked to two of his generals: "I never thought I'd be in this position." His response to 11 September made him even more the object of fascination and respect abroad. Only Churchill and Thatcher have been more celebrated in the United States.

The truly great Prime Ministers are those who execute permanent shifts to the centre of political gravity at home. From the minimum wage to independence for the Bank of England, the Tories have been compelled to embrace New Labour reforms. It was extraordinary that at the first Question Time since the Budget, not one Tory, not a single one, rose to attack the tax rises. Labour was once damned to opposition for advocating spending; now the Conservatives fear to articulate the case for tax cuts. That is a remarkable shift in the terms of political trade between Right and Left.

This adds up to a substantial body of achievement for a man who will only be 49 next week. What is equally striking is that after half a decade at Number 10 there is still a great deal of unfinished business. Only belatedly has New Labour started to address its promise to produce radical improvements to the health service. Blair has rebuilt Britain's relations with Europe, but not persuaded the country that her destiny must be as a fully engaged partner. He has yet to convince the public - he has not even won over the smouldering road block that is his Chancellor - to his own view that Britain's future must be located inside the single currency. That is the most important outstanding challenge of his premiership.

He has made it harder for himself to fulfil that mission by draining away much of the hope and trust that swept him to power. Thanks to the ugly sleaze and spin sisters, the promise that he would "restore faith in public life" is the most conspicuously ruined of his pledges. Even after five years, Mr Blair struggles to quit the damaging addiction to headlinechasing, ill- considered gimmickry such as the widely ridiculed idea of punishing-delinquent children by cutting benefit to their families.

A Prime Minister who often seems to live for the day is paradoxically becoming a leader of unusual longevity. Anyone who tells you that they know for sure when he will depart Number 10 is either a fantasist or a liar. He still has no defined idea himself. It has always been part of the Blair self-conceit that he is not a power-obsessive: he could always walk away. Myself, I find it hard to believe. More importantly, so do those closest to him. Says one long-standing intimate: "He shows no sign I've seen of being bored or tired. If you are searching the crystal ball for a retirement date, I'd be looking very long-term." The often touted idea that he wants to quit Number 10 for Brussels is risible. Who would you rather be: Romano Prodi or Tony Blair? His old friend Bill Clinton, reduced from being the most powerful man on the planet to a lecture circuit celebrity, has warned him about the emptiness of life after office.

Were I Gordon Brown, I wouldn't be putting a deposit on a removal van. All the other Prime Ministers of this Queen's reign left Number 10 because they were ill, ejected by their party or rejected by the voters. Not one went entirely voluntarily. After five years at Number 10, my best guess is that Tony Blair wants at least five years more.