A Lucian Freud portrait hailed as the “turning point” in his career – and which once belonged to the widow of 007 creator Ian Fleming – is to be sold for more than £3.5 million.

The painting is the only single portrait of Freud’s ex-wife, Suzy Boyt.

It is valued higher – inch for inch – than Freud’s record-breaking “Big Sue” which sold for £17.2 million in 2008. That work, properly called Benefits Supervisor Sleeping, set a new benchmark for the highest price ever paid for a work by a living artist.

The portrait of Boyt – mother to five of Freud’s children – is entitled Woman Smiling and will be sold by Christie’s in London in June.

The esteemed art critic Robert Hughes said the painting was “the turning point in Freud’s work with the human clay, when he moved decisively away from the Ingriste modulation of flatness by contour”.

The painting, completed in 1959, was bought by Fleming’s wife, Anne, who was an early champion of Freud’s work.

Its current estimate of £3.5 million to £4.5 million is more than 700 times greater than the price she sold it for in 1973, which was just over £5,000. The 28x22-inch painting has been in a European private collection since 1985. Francis Outred, head of post-war and contemporary art at Christie’s Europe, said: “Lucian Freud is not only the world’s most valuable living artist, but is celebrated the world over as one of the artistic giants of the post-war era.

“This is the work that pioneered the style of painting for which he is most praised and recognised, using thick, expressionist brushstrokes and swathes of impasto to build a human physicality.

“Freud has here literally sculpted in paint the flickering smile into an enigmatic, fleeting presence.”

At the same sale on June 28, Christie’s will also be auctioning five Freud drawings from the collection of Kay Saatchi.

Woman Smiling and the drawings were to be exhibited in Moscow this weekend – the first significant exhibition of works by Freud to take place in Russia.