Filmmakers Ken Loach and Mike Leigh have condemned the Natural History Museum for collaborating with an Israeli firm on a research project.

They were joined by Liberal Democrat peer Jenny Tonge and a number of other critics of Israel in writing a letter to the Independent newspaper.

The signatories complained that the museum was guilty of breaking international law by working with cosmetics company Ahava on a research project into the possible risks of "nanomaterial" (microscope man-made cells which could be used to treat illness in the future).

Other partners in the European Commission-approved research project "Nanoretox", which has been running since 2008, include King's College London, Imperial College London, and universities from Spain, Italy, Denmark, France, and Belgium.

Protesters claim that Ahava products are manufactured in a factory in Israeli settlement, Mitzpe Shalom in the West Bank. The letter said that "organisations which aid and abet this process may well themselves be found to be in violation" of international law.

"We find it almost inconceivable that a national institution of the status of the Natural History Museum should have put itself in this position."

The company has been the subject of a long-running boycott campaign. Last year Ahava closed its central London store following years of weekly demonstrations outside.

In December the National Union of Students voted to condemn King's College for its involvement in the project.