Campaign groups demanded today that the coalition government acts to end the illegal detention of the last British national in Guantanamo Bay.
Former London resident Shaker Aamer is in his ninth year of imprisonment without trial despite being cleared for release by the Bush administration.
During that time he has allegedly suffered horrific torture at the hands of his US captors.
The Save Shaker Aamer Campaign (SSAC) has written to MPs, the Prime Minister and the Gibson detainee inquiry, saying that it would be a "travesty of justice" if Aamer were not able to give evidence to the inquiry regarding the alleged role of MI5 and MI6 in his abuse.
The government has previously made representations for Mr Aamer's release and return to Britain but the US administration is refusing to release him.
The SSAC and other human rights groups have lobbied the US and British government on the issue.
In the letter, the SSAC said: "The detainee inquiry into allegations that the UK government was complicit in torture abroad is due to begin this autumn.
"The terms of reference declare that the inquiry team will speak to all key witnesses.
"Shaker Aamer has stated that he is a victim of torture in the presence of UK agents. He is a key witness for the inquiry.
"It would be a travesty of justice, if he remains in Guantanamo in denial of his right to be heard."
The Director of Public Prosecutions instructed the Metropolitan Police in February 2010 to investigate the allegations made by Shaker Aamer that security services were complicit in his torture.
The Met has yet to complete its report into the claims.