Human rights rulings make the UK a "safe haven" for suspected foreign terrorists, the independent reviewer of anti-terror laws said today.
Lord Carlile said the rejection of the Government's argument that the risk of ill-treatment of foreign nationals deported on suspicion of being involved in terrorism needed to be balanced against the threat they posed if they were to remain, caused problems for the UK.
"The effect is to make the UK a safe haven for some individuals whose determination is to damage the UK and its citizens - hardly a satisfactory situation save for the purist," he said.
In his annual review of counter-terror legislation published today, Lord Carlile backed the Government's attempts to deport suspected foreign terrorists with assurances over their treatment once returned home.
But he warned that it was a "time-consuming process, requiring assurances that are public, credible and reliable".
Even once agreed, "there is no guarantee that the courts will accept them, given the relatively low legal threshold required for an individual to avoid deportation", he said.
The Government has argued that, where a person seeks to resist removal on the grounds of the risk of ill-treatment in their home country, this may be balanced against the threat they pose to national security if they remain.
It also argued that, if the person poses a risk to national security, this should affect the standard to which he must establish the risk of ill-treatment.
But both arguments were rejected by the European Court of Human Rights, Lord Carlile said.
"This leaves the UK reliant on DWA (deportation with assurances) arrangements."
Arrangements currently exist with Algeria, Jordan, Ethiopia, Libya and Lebanon, although the latter may need to be renegotiated "in the light of political developments there", he said.
Home Secretary Theresa May last week outlined plans for a stronger effort to deport foreign nationals involved in terrorist activity.
On control orders, Lord Carlile said the proposed replacement - terrorism prevention and investigation measures, or Tpims - "shares several characteristics with control orders (and would provide commensurate protection)".
"There is an acceptable balance of risk against other considerations," he said.
"It should be seen as adopting a new approach to public protection against terrorism."
Lord Carlile went on: "In stark terms, the potential cost of losing control orders now is that the UK would be more vulnerable to a successful terrorist attack."
But he added that, in future, at least two members of the Opposition should undergo developed vetting and should be given "detailed knowledge of the evidence base for control orders, generally and in relation to individuals".
"The purpose of this would be that, whilst respecting confidentiality and national security, they should be able to give informed advice to their shadow colleagues on the merits of the legislation," he said.
He added that control orders, or their replacement, should only be used when prosecution was not possible and added that it was "unlikely" that the use of intercept evidence in court would have led to the prosecution of any controlees since control orders were introduced in 2005.
Giving members of the Opposition detailed information about the evidence used to justify control orders "would take some of the political steam out of what at times has been a poorly informed debate", Lord Carlile said.
Human rights campaign group Amnesty International said it was "outrageous" to suggest that the UK had become a "safe haven for terrorists", adding that the UK has "some of the harshest counter-terrorism legislation in Europe".
Kate Allen, the charity's UK director, said: "Lord Carlile should remember that if someone is planning a terrorist atrocity in the UK, they can be tried and sent to prison for a long time.
"That is the best way to deal with people who want to murder and maim innocent people - not trying to ship them off, without a trial, to countries where they face a real risk of torture.
"The global ban on deporting people to countries where they're at risk of torture exists for a very good reason - to protect us all from the threat of being tortured."
She went on: "Signing unenforceable 'diplomatic assurances' with countries known to use torture already undermines the international ban on this abhorrent practice.
"The UK should be supporting global efforts to eradicate torture, not trying to get round international law."