MoUs are used to assure governments deportees would not be tortured
Human rights campaigners have accused the government of aggressively pursuing "no torture" deals in order to deport people deemed a security threat.
Amnesty International said the practice of returning suspected terrorists to certain countries that promised humane treatment was a "failed experiment".
It said such promises were "unreliable" and "unenforceable".
The government said it did not deport people to countries where there was a "significant risk" of torture.
The report claims governments are using diplomatic assurances in their "own self-interest to rid themselves of foreigners alleged to be involved in acts of terrorism, instead of prosecuting those persons for any crimes of which they are accused."
Amnesty has called upon the European Union to put a stop to the practice.
Amnesty also criticised the Foreign Office for negotiating "memorandums of understanding" (MoU) with countries including Jordan, Lebanon, Libya and Ethiopia.
The report comes as the Special Immigration Appeals Commission is set to consider the case this week of an Ethiopian threatened with deportation based on an MoU promising the man would not be tortured on return.
But Amnesty said it had serious concerns based on Ethiopia's "exceedingly poor human rights record".
Amnesty International's expert on counter-terrorism and human rights in Europe, Julia Hall, said: "The best way to prevent torture is to refuse to send people to places where they risk being harmed.
"European governments must recommit to the fundamentals of human rights protection. That means safeguarding people from abuse by abiding by their international obligations.
"Diplomatic assurances do not provide such a safeguard and the practice of relying on them should be abandoned."
A Foreign Office spokesperson said MoUs were used as framework agreements where the government seeks specific assurances before returning a suspected terrorist to their country of origin.
The spokesperson added: "The British Government will not deport a person where there are substantial grounds for believing there is a real risk of torture or other cruel, inhuman or degrading treatment, or that the death penalty will apply."
"We have a responsibility to take action to minimise the threat from terrorism to the general public and to consider all options for doing so.
"Both our domestic UK courts and the European Court of Human Rights have accepted the principle of seeking diplomatic assurances in order to remove someone."
The Liberal Democrats said they do not support MoUs if they are unenforceable as "they would be thrown out by the courts" so other arrangements would have to reached.
Lib Dem home affairs spokesman Chris Huhne said: "People are innocent until they are proved guilty in this country, and deporting suspects to torture-risk countries is against British traditions."