Facing fame at Portrait Gallery's Mad Hatter tea party

The most famous faces in Britain will encounter themselves, each other and the Queen at an extraordinary royal tea party organised by the National Portrait Gallery next month.

The guest list for the party, to celebrate the gallery's £16m extension, threatens to turn a sedate afternoon gathering into an event worthy of the Mad Hatter.

Living figures from the arts, business and politics who feature in one of its new galleries - representing the years 1960 to 1990 - have been invited to attend, including long-standing rivals.

Baroness Thatcher has accepted, but so have arch-critic and Tory leadership predecessor, Sir Edward Heath, and long-time opponent Michael Heseltine, whose bid for the Tory leadership ended her career as Prime Minister. The militant miners' leader Arthur Scargill has yet to respond.

Julie Burchill, the opinionated columnist, will be on hand for acerbic observations, leaving other guests to oil the wheels of conversation.

Julie Christie, the actress, hopes to turn up if she can escape her filming duties. Hanif Kureishi, the author, and Sir Alec Guinness, the distinguished actor, have also said they will join the fun. Fashion designer Zandra Rhodes is expected to add some colour. Artist Maggi Hambling, who recently gave one of her paintings for sale to fund Ken Livingstone's mayoral campaign, has accepted, but was shocked to discover she will have to rub shoulders with the Tories.

"I would rather meet the Queen than [Lady] Thatcher," she said, adding: "I always enjoy meeting people shorter than me."

Artists David Hockney and Bridget Riley are also on the invitation list and Helmut Newton, photographer of Baroness Thatcher but more commonly associated with pictures of woman wearing fewer clothes, intends to attend. Sixties pop artist Peter Blake, whose self-portrait is in the new gallery, is expected, too.

Charles Saumarez Smith, the National Portrait Gallery's director, said: "We wanted to give our opening something that was distinctive and showed the special nature of an institution which is pre-eminently biographical." He said there were many arts projects opening in May, including the Tate and the Dulwich Picture Gallery in south London, and there was a danger another would go unrecognised.

This was particularly a risk for the Portrait Gallery, because there is no new public facade; the development uses internal space released to the NPG by the neighbouring National Gallery.

The extension will provide an extra 50 per cent of space and a home for works that include the gallery's distinguished Tudor collection.

The Queen will again see the 1969 portrait of her by Pietro Annigoni when she visits next month, but in the new galleries she will also find four different images of her 16th-century predecessor, Elizabeth I, and a large smattering of other regal ancestors including Henrys VI, VII and VIII, Richard III and Edward IV. One of the aims of the extension is to persuade more visitors to explore further than the ground floor where paintings and photographs of contemporary British famous names are housed.

Dr Saumarez Smith said only 20 per cent of visitors had reached the historic collections of the National Portrait Gallery in the past.

But by placing a restaurant at the top of the new extension with a view across Trafalgar Square to Westminster, he hoped more people would be drawn to the top and then discover the older parts of the gallery's holdings on their way down. "The idea was to spread the vitality of public interest through the collection as a whole," he said.

The opening event will add more images to the store. Each sitter and artist will be invited to stand in front of their respective work. Then they will be photographed.