A portrait by Lucian Freud is expected to set a record for the sale of one of his works.

The painting of Bruce Bernard, a close friend of the artist and brother of the journalist and drinker Jeffrey, is expected to make up to £5.5 million.

The record for a work by Freud is just over £4 million.

The portrait will be sold at Christie's next month as part of a valuable collection of works by the so-called School of London artists, who also include Frank Auerbach and the late Francis Bacon.

The works are being sold by American collectors Elaine and Melvin Merians.

The couple bought their first Freud in London in 1978 and regularly travelled to Britain to buy more works by School of London artists over the next 25 years.

Pilar Ordovas, head of post-war and contemporary art at Christie's, said: "This is the most important collection of works by the School of London artists to appear on the international art market."

Freud's portrait of Bernard was painted in 1992, eight years before the subject died. Bernard, who was at one time picture editor of the Sunday Times magazine, wrote a number of books on photography and one of the most important works on Freud.

The record of £4.152 million for a Freud was set by Red Haired Man On A Chair (1962-63) at Christie's in February 2005.

Freud, the 84-year-old grandson of Sigmund Freud, was born in Berlin and came to Britain with his family when he was 11.

Interest in his work has grown and many of his paintings are sold to America as soon as they are complete, although he has taken to lending them for public display briefly before they go.

He works obsessively at his studio in London, often until the early hours, but has still found time for a string of relationships, often with much younger women.

His children include fashion designer Bella Freud and writer Esther Freud.