Liberal Democrat leader Sir Menzies Campbell has called on Tony Blair to say whether he plans to persuade the US to close the Guantanamo Bay camp.
The US is holding almost 500 terror suspects at Guantanamo Bay
He has asked Mr Blair if he agrees with the Attorney General's view that the prison's existence is "unacceptable".
Lord Goldsmith's remarks on Wednesday were the strongest criticism of the camp yet from a UK government minister.
A senior official at the Pentagon has said the camp will stay open while its 490 detainees remain threats to the US.
"The position in the United States is that as combatants we are entitled as a matter of the law of war to detain them in order to keep them from engaging in the fight against us," said Bryan Del Monte, deputy director of the Office of Detainee Affairs.
Sir Menzies wrote to the Prime Minister: "Could you tell me whether you agree with the Attorney General's view that Guantanamo is a symbol of injustice which must be closed, and whether the government has made representations to the Untied States to this effect?"
He added: "What diplomatic steps will you or your officials be taking in order to press the United States to close the camp?"
Camp 'undermines confidence'
Sir Menzies also said the continued existence of Guantanamo Bay was "undermining international confidence in the commitment of the United States to respect for human rights and the international rule of law".
In February, Mr Blair branded the camp - which opened at a US base in Cuba in 2002 - an "anomaly".
But in the strongest criticism yet from a UK government minister, Lord Goldsmith said on Wednesday Guantanamo tarnished the image of the US.
"The historic tradition of the United States as a beacon of freedom, liberty and of justice deserves the removal of this symbol," he said.
Lord Goldsmith is said to have serious doubts over whether the indefinite detention of "enemy combatants" is legal or fair.
There has been international criticism of conditions and the length of time people have been held without trial.
Rights groups have said the prisoners, held on suspicion of involvement in terrorism, are mistreated through cruel interrogation methods, a charge the US denies.
Nine British nationals at Guantanamo Bay were returned to the UK in 2004 and 2005 after government intervention.