A judge and several leading artists have written to the director of the National Portrait Gallery on the eve of its annual awards calling on it to end its links to BP.

They accuse the central London gallery of helping to launder the oil industry’s image through its BP sponsorship deal and say the oil company is aggravating the climate crisis by extracting fossil fuels.

“Either we distance ourselves from one of the world’s biggest fossil fuel producers and embrace the challenge of decarbonising, or we continue to give legitimacy to BP and its business activities that are seriously exacerbating the problem,” the judge and artist Gary Hume writes in a letter to the gallery’s director, Nicholas Cullinan, seen by the Guardian.

In a separate letter to Cullinan, eight leading artists, who have all been involved with the BP portrait award in the past, have added their voices to calls for the gallery to sever its ties. Those who have signed the letter include Paul Benney, Henry Christian-Slane, Raoul Martinez and Darvish Fakhr.

“We are in a climate emergency,” they write. “Evidence of the damage fossil fuels cause to the climate, and especially to poor, marginalised and vulnerable communities worldwide, is irrefutable.”

The letters are part of a growing campaign against fossil fuel companies’ involvement in the arts, organised in part by the group Culture Unstained.

Jess Worth, the group’s co-director, said: “Art enriches our understanding and experience of the world. BP, with its plan to invest billions in new fossil fuels, is clearly intent on destroying it. By siding with the oil industry rather than the artists and art he is supposed to champion, Nicholas Cullinan seems determined to damage not only the gallery’s reputation but his own.”

Hundreds of people occupied the British Museum in February in protest against its relationship with BP, and protesters say their campaign against “big oil”, and in particular BP, will be stepped up in the coming weeks.

Activists from Extinction Rebellion (XR) are due to disrupt the Royal Opera House’s BP Big Screen event in Trafalgar Square on Tuesday, and the XR families group will stage a protest at the Petroleum Group annual awards dinner, which is due to be held at the National History Museum later this month.

Farhana Yamin, a lawyer who helped negotiate the Paris climate agreement and now coordinates XR’s International Solidarity Network, said big oil companies were in protesters’ sights.

“The environmental movement is coming together and the pressure is relentless. Society will no longer normalise the actions of the oil majors,” she writes in a comment piece in the Guardian. BP “used to be a British state-owned company and it extends our emissions around the world. It must be stopped. Wherever BP goes, protest will follow.”

The National Portrait Gallery (NPG) said in statement that it respected the rights of other people to express their views. “The sponsorship of the portrait award by BP is now in its 30th year and this support directly encourages the work of talented artists and helps gain wider recognition for them and enables free admission for the public, over 275,000 visitors in London last year,” it said. “Since 2010 BP has also given special support to the BP portrait award: next generation programme, which encourages 14 to 21-year-olds to become involved in painted portraiture.”

A spokesperson for BP said the firm was disappointed that several artists had voiced their concerns about its support of artistic endeavours, adding that millions of people had visited exhibitions sponsored by the company.

In a statement it said: “As for BP’s role in the energy transition, our position is clear. We support the Paris agreement and are taking action to advance the world’s transition to a low-carbon future. We’re reducing emissions from our own operations, down 1.7m tonnes last year, improving our products to support our customers’ efforts to reduce their emissions and creating new low-carbon businesses. We are committed to being part of the solution to the climate challenge facing all of us.”

In his letter Hume, however, said that as the climate crisis escalated, it was time for the gallery to cut links with the fossil fuel company.

“As the impacts of climate change become increasingly apparent, the gallery will look more and more out of step by hosting an oil-branded art prize,” he said. “Continuing to promote BP as the climate crisis intensifies will do unacceptable damage to the NPG’s reputation, relationships and public trust. I urge you to commit now to finding an alternative.”