London's biggest advert hoarding

Once it was the giant billboard for Ken Livingstone's publicity war against Margaret Thatcher, with humiliating banners showing the less-than-impressive unemployment figures of her tenure.

Now, nearly 20 years later, the front of London's former County Hall may once again be used for advertising - but this time controversy is looming before anything has even been put up.

The building's owner, Japanese property giant Shirayama, is planning a year-long clean-up of the facade and wants to offer it up for huge "shroud" adverts to be hung on the scaffolding. With nearly 3,500 square metres overlooking the Thames, it is thought firms would pay several hundred thousand pounds a week for what would be one of London's prime advertising spaces.

Such a plan would never normally be allowed on a Grade II listed building, let alone one of County Hall's prominence. But conservation watchdog English Heritage, which is anxious to see the building restored to pristine condition, has made the rare concession that it will not object to advertising in principle if it helps pay the bill.

Instead, English Heritage has simply stipulated that any posters must be "in keeping with the historic area" - which, it says, rules out the prospect of County Hall disappearing under giant Nike emblems or a sea of Tango-coloured orange.

However, no matter how tasteful the designs, the proposal is likely to meet objections from Westminster council, which fears that huge "scaffoltising" ads are becoming far too common across central London. "Our policy is generally against this sort of thing," said a spokeswoman.

A spokeswoman for English Heritage said: "We have been informally approached. While we have expressed caution about what images are used, we have not ruled it out. If it was a tasteful design, we would look at it."

English Heritage has already been in discussion with ad agency Mega Profile, which specialises in large adverts and was behind the giant Versace poster at the Harvey Nichols store in Knightsbridge last year.

Agency managing director Harry Torrance pointed out that scaffoltising is widely accepted as an innovative advertising medium in cities such as Barcelona and Paris. "It's a major step forward for English Heritage," he said. "It's much better to have a well designed advert than a load of scaffolding, especially if it helps pay for cleaning.

"We are talking with a number of major brands about creating something suitable. It is a real challenge to get something that will appeal commercially to sponsors but will be acceptable from a planning point of view.

"Acceptable proposals might feature a work of art, perhaps, with the company logo kept fairly small." A few potential designs are already on the drawing board, although two companies which expressed an early interest - Apple and Ford - have since pulled out.

Insiders say the kind of advert most likely to get approval would probably be for a charity or museum, or for a gallery exhibition, but it is thought that big brand names would be able to offer far more money, even if their logo was kept discreet.

If any such posters do go up on the former Greater London Council HQ, now a Marriott hotel, they will be the first authorised banners since the days when Mr Livingstone ran it as an alternative power base to Westminster Palace across the river.

Among the slogans that taunted Mrs Thatcher were ones declaring London a Nuclear-Free Zone, and - prior to the GLC's eventual abolition - "For Sale".

Ironically, the proposals may once again spark a Left-Right battle across the river. While Tory-run Westminster, whose residents will see the hoarding, is likely to oppose it, the final decision on planning will rest with Labour-run Lambeth, on whose land County Hall stands.

While Lambeth is obliged to take Westminster's opinions into account, a spokeswoman for the council said it would probably go along with English Heritage's final view.