A painting by British street artist Banksy has sold for a record £16.75 million (€19.4m), with proceeds going to benefit state health service staff.

The painting, "Game Changer", sold to an unnamed buyer for £14.4 million plus costs - the highest ever for a Banksy painting - after fierce bidding at Christie's auction house in London.

The signed oil painting by the elusive artist shows a boy holding aloft a nurse doll while Batman and Superman figures are consigned to a bin.

It first appeared at Southampton General Hospital on England's south coast, during the first wave of the global coronavirus crisis in May last year.

The artist - who first emerged out of Bristol's graffiti scene in southwest England in the 1990s - left a note with the painting thanking hospital staff for their work battling the pandemic.

"Thanks for all you're doing. I hope this brightens up the place a bit, even if it's only in black and white," he wrote.

Proceeds from the auction will go to NHS charities and a reproduction of the piece will continue to hang at the Southampton hospital.

The auctioneer hailed Banksy's "incredible gesture" in donating the work.

"It is a Game Changer in every sense," he said of the painting.

The auction price swiftly exceeded the pre-sale estimate of between £2.5m and £3.5m.

Masked intermediaries talking into mobile phones relayed bids from around the world during the sale.

The previous record auction price for a Banksy work in October 2019 was nearly £9.9 million at Sotheby's auction house for "Devolved Parliament", a painting showing parliament populated by chimpanzees.

Christie's said in a statement the painting was a departure from Banksy's usual irreverent style and was instead "a personal tribute to those who continue to turn the tide of the pandemic".

"As an artwork, however, it will remain forever a symbol of its time: a reminder of the world's real game changers, and of the vital work they perform", the international art seller said. 

The painting may also have a political subtext though, as it was donated shortly after Prime Minister Boris Johnson told the public antibody tests could be a "game changer" in the pandemic - only to have experts quickly pour cold water on the idea.

Speaking in a video shared by Southampton Hospitals Charity earlier this month, staff at the Southampton hospital thanked Banksy for the painting and shared their enthusiasm for the work.

Steph Gurney, a senior nurse with the hospital's Acute Medical Unit, said the appearance of the painting had come as a "real shock and a surprise" and that it "meant so much that a world-renowned artist would want to donate a piece of work to our hospital".

Banksy has continued to create other works during the pandemic. In July 2020, disguised as a cleaner, he spray-painted a series of rats encouraging people to wear face masks on the London underground transport system. 

In December, he drew a woman sneezing on the side of a house in Bristol.