Freud's intimate portrait of his friend Bacon sold for £5.4m

A rarely seen oil portrait of the artist Francis Bacon, painted by his friend Lucian Freud, has been sold for £5.4m. The work, which offers an intimate glimpse into the collaborative friendship of two giants of post-war art, is one of only two oil portraits of Bacon painted by Freud and the last remaining: the second was stolen from an exhibition in Berlin in 1988.

The portrait, estimated to sell for between £5m to £7m at Christie's auction house in London, was painted in 1956 and shows the artist with a downward gaze. Bacon, who sat knee-to-knee with Freud while he worked on the painting, is said to have "grumbled but sat consistently" during the first six months of sitting, according to Christie's. It is thought Bacon left suddenly, most likely to pursue his lover, Peter Lacy, in Tangiers.

Although it remained unfinished, art critics agree it offers a snapshot into the working methods of the younger artist at a critical point of his development; Freud had begun to work in a more expansive way, using thicker brushstrokes, liberating the paint and creating a more worked complexion, more seasoned and full of life.

The portrait was part of Christie's sale of post-war and contemporary art. The auction may come to mark a turning point in the fortunes of the art market, which has until now defied the economic downturn. Of 47 lots, only 26 were sold and the sale made £32m, against pre-auction estimates of £57m to £75m.

By contrast, the record for a painting by Freud was set at Christie's in May, with his naked, large-scale work, Benefit Supervisor Sleeping, selling for £17.3m. The record for a Bacon work stands at £43m, for Triptych, created in 1976.

The Bacon portrait sold yesterday was acquired in 1972 by a private collector from a London gallery and had remained in the same hands since.

Graham Sutherland, a mutual friend and artist, introduced Freud to Bacon in 1945, inviting them to his house. They formed a close friendship and saw much of one another in the following years.

Bacon had a great influence on the younger Freud and is often credited with liberating his style and fuelling his desire to depict human life.

In the early 1950s, the artists sat for each other; Bacon's first portrait of Freud came in 1951, and many others followed. Freud painted Bacon just twice.