The law may have to be changed to deport Islamic extremists to countries where they may face torture or inhumane treatment, Tony Blair has said.
London bomber Khan criticised British foreign policy
The government has been seeking assurances from several countries that no-one it deports will be ill-treated.
The European Convention on Human Rights currently blocks the UK from sending back foreigners considered to be promoting terrorism.
But Mr Blair told the BBC: "We may have to amend the law to get it done."
In an interview in Beijing during trade talks, Mr Blair was asked why no deportations had yet taken place.
He replied: "You can serve the orders for deportation swiftly, but the legal process then takes some time to determine.
"Because there's a major question, which is whether we are going to be able, on the basis of understandings with the countries to which we're returning these people, whether we will be able, according to the law of the courts, to be able to return those people properly.
"And that's the issue which I explained, you know before I left, and that's why this will take some time through the courts, through the Parliament, because we may have to amend the law in order to get it done."
But shadow home secretary David Davis said that while Mr Blair was "very good on stern tones", the issue was whether he could deliver on a change in the law.
He said he had warned Home Secretary Charles
Clarke at the time when changes to the rules on deportations were announced that the wording of planned legislation would have to be revised in order to
overcome difficulties in the courts.
"We have been saying for years now that these people should be deported," Mr Davis told BBC Radio 4's Today programme.
"We have called for either reform or repeal
of the Human Rights Act. We have called for action to ensure that British courts behave in a similar way to some European courts on all this.
"Of course, it is now going to take time. They have waited until now to do something about it and it is going to take time. They will get our support in terms of doing that, but there will have to be changes in the law."
The two politicians were speaking just days after a video was shown on Arab TV network al-Jazeera showing London bomber Mohammed Sidique Kahn criticising British foreign policy and saying he was a soldier fighting a war.
Commenting on the video, the prime minister said: "In the end, we have got to challenge the idea that a section of our country is being victimised by our country."
He said the government had to "take on the ideas of these people, to challenge them head on - to say it's absurd of anybody growing up in our country to say they are a victim or have a grievance on these issues."