Bacon kept the painting for two years before selling it

Three Studies, painted in 1980 when Bacon was 71, is expected to fetch up to �5.5m when it is auctioned at Christie's in London 22 June.

The triptych has never been seen before by the public.

It was painted at a time when Bacon became concerned for his own mortality as he entered old age.

Several of his friends had also died in the past decade, which increased his fear - his partner George Dyer committed suicide in 1970.

Private sale

A portrait of him by Bacon fetched �4.9m at auction last year.

Dublin-born Bacon often dealt with themes of death and decay in his work and is probably most famous for his portraits.

Bacon kept hold of Three Studies for two years before selling it himself rather than through his gallery.

He once said: "I don't know that I should talk about a triptych in my case.

"Of course, there are three canvases, and you can link that to a long-standing tradition. The primitives often used the triptych format, but as far as my work is concerned, a triptych corresponds more to the idea of a succession of images on film."

The triptych is being sold as part of Christie's Post-war and Contemporary Art Sale.

Other works on offer include two paintings by Jean-Michel Basquiat and one by Andy Warhol titled The American Indian.

The entire sale is expected to fetch in the region of �21m.