The government has been accused of "actively trying to undermine" the global ban on torture in its efforts to tackle terrorism.
Campaign group Human Rights Watch says efforts to return suspected terrorists to countries where they might be tortured are in direct contradiction with the government's public opposition to torture.
However, the Home Office has rejected the report, saying that although it agreed that people should not be sent back to countries where they may face torture, their approach to get around this was backed up by the courts.
A spokesman also insisted there was "no truth" in the claim that a new law was being planned for the Queen's speech later this month that would allow the government and courts to balance the risk of torture against national security.
Britain is prevented, under human rights law, from deporting people to countries where they may be ill-treated. As a result, it has signed "memoranda of understanding" with Jordan, Libya, the Lebanon and others to safeguard the rights of anyone sent back.
Critics have argued these assurances are not worth the paper they are written on, and today Human Rights Watch associate director Benjamin Ward warned: "When it comes to torture, diplomatic assurances simply don't work.
"Putting them in a 'memorandum of understanding' and adding post-return monitoring does nothing to change that."
However, the Home Office told politics.co.uk that although it agreed that no-one should be returned to a country they might face torture, it believed the assurances were working - and was happy for the courts to decide whether they were on a case by case basis.
A spokeswoman noted the ruling of the special immigration appeals commission (SIAC) in August that a suspected terrorist could be returned to Algeria without breaching his human rights, because of assurances from the Algerian government.
Human Rights Watch also cites as a matter for concern the government's challenge to the 1996 European court ruling that Karamjit Singh Chahal, a Sikh militant, could not be sent back to India for fear of being persecuted for his political views.
The court ruled that in deciding deportation cases, judges could not balance the right of an individual not to be tortured against the interests of national security. Today's report says this ruling was a "key judgment affirming the absolute ban on returns to torture".
"Opening the door to torture won't make Britain safer," said Mr Ward. "The most effective response to terrorism is good police and intelligence work, not setting aside core values."
However, home secretary John Reid has said defendants of the Chalal case "just don't get" the threats facing the UK.
And the Home Office spokeswoman today insisted ministers must be able to consider risks to national security in deportation cases, saying: "This doesn't give legitimacy to torture