Britain has nine nationals held at the Cuban base, all of whom are accused of having links with the al-Qaeda terrorist network in Afghanistan.
Now MEPs are demanding they must have a fair trial or be released.
It has been nearly two years since many of the detainees at Guantanamo Bay had any contact with their families or lawyers.
One of the Britons detained, Moazzam Begg, has been told he will face a military trial, but so far no date has been set for the hearing.
On Tuesday his father, Azmat Begg, made an emotional appeal before a special meeting of MEPs and lobby groups at the European Parliament in Brussels.
He begged the EU to put pressure on the United States either to release his son, or allow him to face trial in Britain.
“They should come forward and help me get my son released from this place where he’s being treated like an animal.
“They have got the right to ask Mr Bush to release the prisoners because we co-operated a lot with them and this is the time to repay,” he said.
MEPs from Sweden, Spain, France and Germany, which also have citizens detained at Guantanamo Bay, said it was time the European Union presented a united front on the issue.
UK MEP Baroness Ludford asked EU states to refuse to sign any new resolution on Iraq until the United States put an end to the “intolerable suffering” of the Guantanamo Bay prisoners.