A radical Muslim cleric accused of posing a grave threat to Britain's national security should be released regardless of the risk he poses, his lawyers insist.

Abu Qatada, who is being held in high security conditions, wants to be released on bail as he fights deportation to Jordan.

But lawyers for home secretary Theresa May are battling to keep him behind bars while British diplomats continue to seek assurances from the Jordanian authorities that evidence gained through torture would not be used against him.

Ed Fitzgerald QC, representing Qatada, told an immigration judge in central London that Qatada had now been held for six-and-a-half years while fighting deportation.

He told the Special Immigration Appeals Commission (Siac) that that was "against a background of almost nine years detention without charges on the grounds of national security".

Mr Fitzgerald said: "The detention has now gone on for too long to be reasonable or lawful and there is no prospect of the detention ending in any reasonable period.

"However the risk of absconding, however the risk of further offending, there comes a point when it's just too long.

"There comes a time when it's just too long, however grave the risks."

Mr Fitzgerald added that the eight-and-a-half years Qatada spent in custody was equivalent to a 17-year jail sentence.

The bail hearing was ordered after Qatada, once described by a Spanish judge as "Osama bin Laden's right-hand man in Europe", won an appeal to the European Court of Human Rights (ECHR) last month.

Human rights judges ruled last month that sending Qatada back to face terror charges without assurances that evidence gained through torture would not be used against him would deny him his right to a fair trial and be a "flagrant denial of justice".

Mrs May vowed that Qatada, who has been held for six-and-a-half years, would be kept behind bars while she considered all legal options to send him back.

The Home Office added that he "poses a real risk to national security".

But Qatada's defence team said he has been kept behind bars for more than six years already and should be released.

The ruling last month was the first time that the Strasbourg-based court has found an extradition would be in violation of Article 6 of the European Convention on Human Rights, the right to a fair trial, which is enshrined in UK law under the Human Rights Act.

But the home secretary has three months to lodge an appeal with the court's Grand Chamber.

The Henry Jackson Society think-tank said the ruling "undermines national security" while former home secretary David Blunkett warned Qatada was "extraordinarily dangerous and we don't want him on our streets".

Qatada, also known as Omar Othman, 51, featured in hate sermons found on videos in the flat of one of the September 11 bombers.

Since 2001, when fears of the domestic terror threat rose in the aftermath of the attacks, he has challenged, and ultimately thwarted, every attempt by the Government to detain and deport him.

Law Lords ruled almost three years ago that he could be sent back to Jordan and Lord Phillips, now president of the Supreme Court - the highest court in the land - said torture in another country does not require the UK "to retain in this country, to the detriment of national security, a terrorist suspect".

But the human rights court went against that judgment, agreeing with the earlier 2008 decision of the Court of Appeal which said there were reasonable grounds for believing he would be denied a fair trial in Jordan.

But Tim Eicke QC, for the home secretary, said Mrs May did not accept that Qatada's detention was unlawful.

The length of detention "has to be weighed against the risks" and "he poses a particularly serious risk to the UK".

"The secretary of state has also taken all steps to diligently try to achieve removal and deportation as soon as possible.

Updated: 12:06pm, 6 February 2012