Lucian Freud appeals for return of his stolen portrait of Francis Bacon

An international hunt to find a stolen portrait by Lucian Freud of his fellow artist Francis Bacon begins today, backed by a giant poster campaign and £100,000 reward.

The aim is to recover the work – only the size of a large postcard but worth at least £1.2m – in time for a retrospective of Freud's work at the Tate Britain gallery in London next year, when the artist is 80.

"Would the person who holds the painting kindly consider allowing me to show it in my exhibition next June?" Freud requested in a statement yesterday.

Nothing has been heard of the painting since it was stolen from a British Council exhibition in Berlin in 1988. Andrea Rose, the council's director of visual arts, described the theft as "the worst moment of my professional career".

Freud designed the "wanted" poster, and 2,500 copies of it will be splashed across Berlin from today. Although the work could be anywhere, Ms Rose said it might still be in the country. A statute of limitations in Germany means no one can be prosecuted for a crime after 12 years.

A private donor has offered a reward of up to £100,000 for information leading to the return of the painting, which had been borrowed from the Tate.

William Feaver, Freud's biographer and the curator of next year's show, said it was the most important small portrait of the 20th century and one of the three crucial paintings of Freud's early career.

It was painted in 1952 and bought by the Tate the same year. It was the only portrait Freud painted of Bacon, who had been his friend since 1944. "It's an extraordinary painting, one of the 20th century icons," Mr Feaver said.

Ms Rose said it was uncommon when paintings disappeared to advertise the fact because they frequently reappeared within two or three years. This case was unusual in that there had been no word since the theft.

The retrospective was the spur to see whether publicity could now help. "It is an Anglo-German collaboration to recover for the nation a major picture of international significance," she said.

The British Council is indemnified by the Government when it borrows works for foreign tours, but no insurance claim has been made to the Treasury because no comparable work with which the Tate could replace it has come on the market.