Speaking to the British parliament on Wednesday, Tony Blair also repeated his opinion that the camp, used to house terror suspects captured by the US, was an "anomaly", but said it was important to remember the context in which the camp was set up.
The prime minister went further than before during his weekly question and answer session in parliament, saying: "I hope the judicial process can be put in place which means that Guantanamo Bay can close - as I think it should for the reasons I have given."
He said that the camp came out of "the worst terrorist act this world has ever known", referring to the attacks on 11 September, 2001, in the United States when nearly 3000 people were killed.
"Those people who were picked up in Afghanistan were people who were engaged in helping those reactionary forces there to defeat American and British troops," Blair said.
"Those people who were picked up in Afghanistan were people who were engaged in helping those reactionary forces there to defeat American and British troops"
Tony Blair, the British prime minister
"So, I agree it is an anomaly and that's why it has to end. But I'm afraid I will always - when answering questions on this - draw attention to the circumstances in which it was introduced."
The first detainees arrived at the US naval base on 11 January, 2002.
In the four years since, some 500 detainees have been held at Guantanamo, but only 10 have been formally charged as alleged terrorists. Most were captured soon after the US-led war in Afghanistan against the Taliban regime.
In Washington meanwhile, a Yemeni man held at Guantanamo has launched a court challenge against the force-feeding of inmates on hunger strike, a lawyer said on Wednesday.
Several inmates went on hunger
strike against prison conditions
The action taken by Mohammed Bawazir alleges that he and other detainees were given treatment amounting to torture to force them to take intravenous feeding.
Rick Murphy, the US lawyer taking the action for Bawazir, said that federal judge Gladys Kessler had given US authorities until late Wednesday to give their version of the facts.
Bawazir has been detained at the camp for suspects held in the US "war on terror" since early 2002. He joined a hunger strike by many of the inmates in August 2005 to protest against the conditions of their detention.
According to US lawyers who have had restricted access to the detainees, US military authorities have recently changed their tactics to make the hunger strikers accept nourishment.
Before January, Bawazir and the other hunger strikers had been force-fed through a tube.
The lawyers said that since 11 January, Bawazir has been forcibly strapped into a chair - with his legs, arms, head and midriff tied to the chair - to be fed.
The US Congress recently passed
a law against torturing detainees
They added that Bawazir was denied access to a toilet while being fed and for an hour or more afterwards.
Documents sent to the court said that Bawazir abandoned his hunger strike on 22 January "only because of brutal treatment that respondents (the US military) began inflicting on 11 January".
The action argues that the tactics being used are against a recent law passed by Congress which banned the use of torture against detainees held by the United States anywhere in the world.
The US authorities have repeatedly insisted that the Guantanamo detainees are treated humanely.