The Prime Minister’s wife posed naked for the artist Euan Uglow when she was an unknown trainee barrister in her mid-twenties.

For more than a year Cherie Booth earned £5 an hour modelling for the 5ft painting Striding Nude, Blue Dress, wearing a sleeveless dress open at the front. She was required to stand still for up to an hour at a time in Uglow’s freezing studios in Clapham, South London. The artist later painted a second version with another model.

Will Darby, of the Browse and Darby gallery in Mayfair, who was Uglow’s close friend and dealer for 25 years, told The Times: “He wasn’t pleased with the painting because it wasn’t finished. We exhibited it in 1983 alongside the finished one but after that he just didn’t want to show it.

“He was friends with the Blairs and it was his wish that it would never be shown after they entered public life.”

Mrs Blair was introduced to Uglow by Derry Irvine, the future Lord Chancellor. She remained friends with the artist and is thought to have named her eldest son after him. As the Blairs’ eminence in political circles grew, Uglow became ever-more reluctant even to acknowledge the painting.

The portrait was stored at Browse and Darby until Uglow’s death in 2000 at the age of 68. It was transferred to the care of the artist’s estate, the executors of which include Lord Irvine, and is now held by the Marlborough Fine Art Trust.

Mr Darby said that it was difficult to put a value on the work: “I don’t think anybody would want it because there is not enough of it there. Perhaps someone with an interest in politics might.” He confirmed that neither version of Striding Nude, Blue Dress would feature in a retrospective of Uglow’s work in May at Browse and Darby.

No 10 declined to comment yesterday.

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