Oil companies are becoming increasingly unfashionable partners for British cultural institutions, as they ditch corporate sponsorship under pressure from artists and environmentalists.

Members of the "BP or not BP?" protest group on Tuesday disrupted an event for the British Museum's new exhibition on Troy, which is sponsored by the British oil giant.

Activists dressed as Greek gods and heroes such as Zeus, Athena and Achilles, were sprayed with a black liquid by "Petroleus" -- a deity created especially for the occasion.

"The company sponsors the museum in order to look like a generous gift-giver that cares about culture but, in reality, it's a cynical attempt to deflect attention from something far more sinister."

A spokeswoman for the museum told AFP she "understands" the concerns.

But temporary exhibitions such as the Troy show were "expensive to stage, and only possible to plan, develop and host with this kind of external support", she said.

The activists, who say they have already held 37 protests at the British Museum, are in no mood to back down. Now they plan to build a Trojan horse and lay siege to the venue.

- Climate emergency -

The activists have already forced some institutions to back down and left others wondering about whether to persist in corporate sponsorship from oil firms.

The prestigious Royal Shakespeare Company, based in the playwright's hometown of Stratford-upon-Avon in central England, announced in October that it was cutting its ties with BP.

BP said it was "disappointed and dismayed" by the decision, pointing out its sponsorship enabled the theatre company to offer cheap tickets for young people.

But it was a letter from a youth group behind school protests that tipped the balance.

"Amidst the climate emergency, which we recognise, young people are now saying clearly to us that the BP sponsorship is putting a barrier between them and their wish to engage with the RSC," the institution's management explained.

"We cannot ignore that message."

Campaigners "BP or not BP?" -- their name is a play on the famous soliloquy in Shakespeare's "Hamlet" -- have been pushing against oil money in the arts for years.

"This year has been the year where it all came to fruition," one of its members, Danny Chivers, told AFP.

"Partly because of movements like the school strikes and Extinction Rebellion," which organised a series of disruptive protests to highlight the urgent need to fight climate change.

Also, "a lot of people inside the art world have been speaking out more and more loudly," he said, citing leading actor Mark Rylance, who quit the RSC because of BP's involvement.

National Galleries Scotland followed suit earlier this month.

"We recognise that we have a responsibility to do all we can to address the climate emergency," it said, explaining its decision.

- Liberate the Louvre -

Activists have promised there will be no let-up. Now they intend to target the Royal Opera House and the National Portrait Gallery both of which still receive sponsorship from BP.

A spokeswoman for the central London gallery told AFP it was "listening carefully to voices on all sides".

In France meanwhile, activists calling themselves "Liberate the Louvre" have mobilised against oil giant Total, performing theatrical events protesting its sponsorship of the museum.

It is not just oil companies who are feeling the heat.

Money from the Sackler family, which built its fortune on the powerful painkiller accused of fuelling the US opiate crisis, is now being ignored by some of the world's greatest museums.

It remains to be seen how institutions will plug the funding gap.

But Chivers said London's Tate, and the Edinburgh International Festival in Scotland both gave up taking oil money in 2016 and "seem to be going just fine".afp