An unfinished “lost” painting of a male nude by Francis Bacon has been discovered on the reverse of two paintings by Irish artist Tony O’Malley which are being auctioned on St Patrick’s Day.

The paintings by Co Kilkenny-born O’Malley were made in the early 1960s. They each measure about 30ins by 47ins.

They will be sold as a pair at Christie’s in London and have an estimate of £20,000- £30,000 (€25,827-€38,753).

O’Malley made the oil paintings using a board which he had cut in half. But the board had earlier been used by Bacon to create the unfinished picture.

The two O’Malley paintings went to different owners but have now been reunited.

When they are turned over and joined up they reveal the Bacon image, which has been assigned the title Figure, circa 1959.

“For years these works [by O’Malley] were separated, residing in the collections of two different owners,” Christie’s said.

“Now these paintings, and the lost Bacon study, will be reunited and viewed together for the first time in almost 60 years.”

The O’Malley paintings are entitled Currach, Clare Island and Evening Landscape, Tehidy Hospital.

In the late 1950s the two men crossed paths in the artists' colony of St Ives in Cornwall, where O'Malley was then living and working.

Christie’s said Bacon moved to St Ives in 1959 but “cut short his visit following an argument” with his partner and “abandoned the work”.

The woman who had rented him a studio gave Bacon’s discarded works to friends who reused the materials.

Reputedly, O’Malley was approached by a dealer who suggested he fraudulently complete the Bacon painting but he refused and split the board in two.

Another artist reputedly used one of Bacon’s paintings to roof a chicken shed.

In demand

Dublin-born Bacon, who spent his working life in England and died in 1992, has, in the past decade, become one of the most expensive painters in art history.

One of his triptychs entitled Three Studies of Lucian Freud sold for $142.4 million (€129.4m) at Christie's, New York, in November 2013 – the second highest price ever paid for a work of art at auction.

A spokesman for Christie’s South Kensington saleroom said the paintings had been “assessed on the basis that they are works by Tony O’Malley”.

Christie's said O'Malley gave Currach, Clare Island to his friend the poet Padraic Fallon but kept Evening Landscape, Tehidy Hospital.

Tehidy is a few miles from St Ives.

O'Malley left St Ives and moved to the Bahamas before returning to Ireland with his wife Jane.

He was elected a Saoi of Aosdána in 1993. He died in 2003.

Barbara Dawson, director of Dublin City Gallery The Hugh Lane, which houses Bacon's reconstituted London studio (which was donated to the State by his heirs) and paintings by O'Malley, said the paintings would make a "very desirable acquisition".

She declined to comment on the possibility of the gallery bidding for them.