It takes three types of people to be the artist’s model, spending hours in rigid poses as eyes scrutinise your body: the day-dreamer, the monstrous egotist or the mathematician, who might puzzle out some algebra to pass the time.

When I sat for Euan Uglow, the artist whose picture of a young Cherie Booth has emerged after two decades locked in a storeroom, it was as day-dreamer, letting my mind drift for hours at a time. I do not know of Cherie’s approach, but she deserves praise for taking on a role that can intimidate, can fascinate and can be the most sensual of experiences.

Uglow was a perfectionist, and I had to suffer somewhat for his art. To be seated in a chair, stripped to the waist, for three hours a day, three days a week, for nearly three months, was no small enterprise. I sat on a hard wooden chair, my palms cupped round my elbows, with a plumb line dropped down in front of me and a spirit-level across my lap. Chalk marks recorded my position precisely while behind my head was a green painted grid.

Uglow was not the most settling of employers, as Cherie may have discovered. He told me he liked “flesh that looks like it’s just crawled out from under a stone”. The Prime Minister’s wife’s pale complexion would no doubt have appealed.

Her picture, revealed for the first time in 23 years after being kept in the storeroom of a London gallery, shows some of the feistiness one might expect of a trainee barrister in her mid-twenties.

While it has been known that Cherie posed for Uglow, only now, six years after the artist’s death, has it been revealed that Striding Nude, Blue Dress is Mrs Blair.

Her distinctive facial profile is clearly recognisable, but the work was never completed because she decided to cut short the sittings to visit the US.

She married Tony Blair, then a trainee barrister colleague, shortly afterwards in 1980. As the pair moved into public life, so Uglow, who exhibited the picture briefly in 1983, decided it was best kept out of view.

The best models, Uglow told me (and I’m sure he threw down the observation as a gauntlet), are either very intelligent or very stupid. They are the ones with the inner resources or the utter vacuity to remain completely still despite all the muscle cramps. They can create a world of their own in their heads. It was the mediocre ones that wriggled and fidgeted, he said. Mrs Blair no doubt belonged in the intelligent category. And the painting that results — which is expected to come on public view in an Uglow retrospective next year — bears testimony not to a passive or exploitative relationship, but to a creative partnership. The poser participates in the making of the work.

What the viewer sees is, quite literally, a life laid down hour by hour, layer by layer, upon a canvas.

On one level the Prime Minister’s future wife stands exposed. We stare at the pale flesh that Uglow so loved in all its prismatic subtlety.

But on another level she is completely unreachable. She is removed by her unique relationship with the painter into the realm of art.

It is as if she has been locked into and out of herself both at the same time.

This is what you must feel to be a good artist’s model.

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