The Science Museum has been accused of trying to “quietly hide away dirty money” after it agreed to a request by the Dr Mortimer and Theresa Sackler Foundation to repurpose a £2m donation earmarked for a prominent new gallery.

The donation, which was meant to fund the Medicine: the Wellcome Galleries collection – dedicated to thousands of medical artefacts, including the world’s first MRI scanner– was removed from the project earlier this year as the Sacklers’ philanthropic donations came under increased scrutiny.

In 2017, the Science Museum announced that the galleries would have two new key funders: GSK and The Dr Mortimer and Theresa Sackler Foundation. But when the galleries opened in November this year there was no mention of the Sacklers on its website and GSK was named as the only principal sponsor.

The museum confirmed to the Guardian it plans to keep the money and spend it on other projects, and said a donor board at the museum names the Dr Mortimer and Theresa Sackler Foundation among its funders.

The Science Museum said it came to an agreement with the foundation that the £2m gift “would be most effectively applied to support work across the museum’s world-class scientific, technological and medical collections rather than being specifically designated to one project”.

The museum has previously been criticised for taking money from BP and Shell. The director of the Science Museum group, Ian Blatchford, defended the museum’s decision to take money from oil companies, including for sponsorship of exhibitions about climate-related issues.

In an email to staff this summer, he acknowledged the challenges surrounding the climate crisis, but said big oil and gas companies “have the capital, geography, people and logistics to find the solutions [to climate change] and demonising them is seriously unproductive”.

The campaign group Culture Unstained said that despite Blatchford’s “regular outbursts in defence of his unethical donors and sponsors”, on this occasion the museum had “accepted that the Sackler name is just too toxic”.

A spokesperson said: “Rather than trying to quietly hide away dirty money, the museum should be drawing an ethical red line and turning it down. When other cultural organisations are confidently cutting their ties to oil money too, it’s troubling that when it comes to ethical sponsorship the Science Museum is still dragging its feet.”

The US pharmaceutical company Purdue Pharma, which is owned by some branches of the Sackler family, makes the highly addictive opioid prescription painkiller OxyContin, and is facing several lawsuits over its alleged role in the US opioid crisis. The branch of the family descended from Arthur Sackler is not linked to OxyContin. The Sacklers, who deny all the allegations, have been significant funders of British culture for many years, and the Science Museum is the latest high-profile institution to have its connections to the family questioned.

In March, the National Portrait Gallery announced it was dropping a £1m grant from the family after pressure from campaign groups which said accepting money would make them complicit in the harm its products have allegedly caused. “They did the right thing,” said Nan Goldin, a US artist who has been one of the Sacklers’ most vocal critics and has previous discussed her own addiction to opioids. “I hope there is a domino effect now; there needs to be.”

Earlier this year, Tate announced it would no longer accept donations from the family because of its alleged links to the opioid crisis. Shortly afterwards the Guggenheim Museum in New York, where a Sackler sat on the board for many years, said it would no longer accept gifts from the family. In July, the director of the Victoria and Albert Museum, Tristram Hunt, defended its decision to not remove the Sackler name from its galleries.

Last year, the South London Gallery returned a £125,000 award from the Sacklers, with its director, Margot Heller, telling the Art Newspaper that “the gallery’s board took a majority decision that it was in the best interests of the charity to return the grant”.

Representatives from the Dr Mortimer and Theresa Sackler Foundation did not respond to a request for comment.