Oil giant BP is cutting its controversial sponsorship of UK cultural institutions by 25 per cent to £7.5m (US$9.9m, €8.9m).

The sum – pledged over a period of five years to the Royal Opera House, British Museum, National Portrait Gallery and Royal Shakespeare Company – will come into effect from 2018.

BP had previously sponsored the Royal Opera House, British Museum, National Portrait Gallery, plus Tate Britain for a combined £10m (US$13.2m, €11.9m) in deals running up to 2017. Earlier this year, BP announced it would be ending its 26-year relationship with Tate Britain in 2017.

A spokesperson for BP said that due to "cutting costs and reducing staff numbers” the company “could not continue all of our sponsorships at the same level".

The renewal of BP’s sponsorship deal has drawn the ire of anti-oil protesters, who have promised to “escalate protests”.

“At a time when the world needs to urgently shift away from fossil fuels, the idea that these institutions will still be promoting an oil company into the 2020s is deeply irresponsible,” said a spokesperson for campaign group BP or Not BP.

“As the devastating effects of climate change becomes ever more apparent, this decision will look more shortsighted with every passing day.”

The sponsorship of oil companies with cultural institutions has long been debated, with concerns raised of the amount of power these companies can wield with their financial influence. Earlier this year, campaign group Art Not Oil published new information on what it described as the “corrupting influence” of BP over national museums and galleries receiving its sponsorship in the UK.

The in-depth report, which drew upon hundreds of emails, documents and correspondence released through the Freedom of Information Act, revealed alleged interference in curatorial decision making and museum security from BP.

BP responded to Art Not Oil’s report, stating it “never seeks curatorial influence”, providing “nothing more than funding” to its select institutions, though some of the documents included seemed to paint a different picture.

All four of the arts organisations released statements on the deal, with new British Museum director Hartwig Fischer saying that BP’s sponsorship had “enabled the museum to host magnificent exhibitions and events with a great public benefit.”

Royal Opera House chief executive Alex Beard added that “BP has been and remains at the forefront of corporate investment in arts and culture in the UK”, while Royal Shakespeare Company executive director Catherine Mallyon said that BP’s sponsorship “helps us establish lifetime enthusiasts for Shakespeare and live theatre.”

Nicholas Cullinan, director of the National Portrait Gallery, called BP’s sponsorship vital in the current financial climate.

“We are extremely grateful for and proud of this ongoing partnership, for such longevity and loyalty is unique in corporate sponsorship,” he said in a statement. “At a time when funding for public institutions is increasingly stretched, the support of the arts that BP provides is vital.”