Pink Floyd rocker Roger Waters, pop star Sophie Ellis-Bextor and actress Juliet Stevenson are among those who have signed an open letter calling for the release of a British man from the Guantanamo Bay detention camp.

Shaker Aamer has been held at the controversial American institution for 13 years without trial, and his ongoing imprisonment has sparked outrage among both celebrities and politicians.

A new letter as part of the We Stand With Shaker campaign has been published in Britain's Daily Mail newspaper and features the signatures of Waters, Ellis-Bextor and Stevenson, as well as a number of leading politicians, journalists and human rights activists.

The note urges U.K. Prime Minister David Cameron to use his relationship with America's President Barack Obama to secure Aamer's release.

It reads, "The British Government has a non-negotiable responsibility to secure the return of Mr Aamer, given his status as a legal British resident. We can find no reason why, given the special relationship between our two countries, the Pm (Prime Minister) cannot call President Obama and tell him that Mr Aamer must be returned to the U.K. as swiftly as possible. We urge Mr Cameron to pick up the phone to President Obama, and to bring Shaker Aamer home.'

A spokesman for Cameron says of the Aamer case, "We will keep pressing it. It is our objective to get him back to the U.K. That decision is in the hands of the U.S. government."