LONDON (Reuters) - Works by Francis Bacon and Andy Warhol will form part of what auctioneers Sotheby's said on Tuesday is poised to be the most valuable sale of contemporary art in London, with a pre-sale estimate of up to 204.6 million pounds ($425.1 million).

The July 1 and 2 sale will be headlined by Bacon's Study for a Pope I estimated at 25 to 35 million pounds and include two of the Irish-born British artist's rediscovered self-portraits.

The Warhol works will be among 21 paintings inspired by the U.S. dollar, including the most important dollar-themed works by the American pop artist in private hands, Sotheby's said.

Bacon's Study for a Pope was painted in 1961, inspired by 17th-century Spanish artist Diego Velázquez's Portrait of Innocent X. It had previously sold at auction in 2005 for $10 million, which was the record price for any work by the artist at the time.

The work expected to fetch the most from the dollar-inspired paintings is Warhol's One Dollar Bill (Silver Certificate) from 1962, the only one of the series he painted entirely by hand, which has a pre-sale estimate of 13 to 18 million pounds.

The dollar collection also features works by Keith Haring, Joseph Beuys, Tim Noble and Sue Webster.

"The dollar is more than just an American currency, it's a symbol of so many things - aspiration, wealth, glamour - and I think (Warhol) was brilliant at pinpointing a trigger which everyone can draw to," Sotheby's Contemporary Art specialist James Sevier said.

He told Reuters Television that Warhol painted the $1 and $2 bills rather than the $50 or $100 "because it was about accessibility, like the Campbell's Soup Cans", referring to another of Warhol's most famous series.

Sotheby's said in a statement: "(The auction) is poised to be the most valuable sale of its kind ever held in London."