LONDON (AP) _ Stamps commemorating Princess Diana went on sale today, with the Post Office predicting they would be the biggest-selling set in history.

The five first-class stamps, which cost 40 cents each, will be the only set issued in Britain with the princess's image on them.

Bordered in purple and carrying the dates of her life, 1961-97, they show formal and informal shots of Diana by photographers Lord Snowdon, Terence Donovan, Tim Graham and John Stillwell.

``Further stamps of the issue have already been reprinted _ the first time this has ever happened for a special issue,'' said John Roberts, the Post Office's chief executive. ``More than six times the normal number of special stamps have been distributed to all 19,000 post offices to meet the expected initial demand.''

On the basis of that anticipated demand, the Post Office has agreed to an initial donation of at least $10 million to the Diana, Princess of Wales Memorial Fund. All further profits also will go to the charity.

The Royal Mail already is printing extra supplies and will keep the stamps on sale throughout 1998. They also will be available from the British Philatelic Bureau in Edinburgh, Scotland, and from stamp agents around the world.

Also available is a commemorative cover, a pictorial envelope that can be specifically postmarked and will be on sale only until Feb. 28. It features a photograph of Diana on an April 1985 royal tour of Italy, wearing a pink suit and matching hat, and carrying flowers.

The village post office in Great Brington, Northamptonshire, where Diana grew up and used to shop, is issuing its own first-day covers of the stamps. Postmistress Christine Whiley said 10,000 first-day sets would be available at $16 each.

The only other British stamp ever to carry Diana's image was issued in July 1981 to celebrate her marriage to Prince Charles.