The damning verdict on the deportation proposals, from Manfred Novak, the UN's special rapporteur on torture, came as the Government's problems deepened over its anti-terror crackdown.
Ministers have been examining ways to prevent the extremist cleric Sheikh Omar Bakri Mohammed from returning to Britain from Lebanon and have abandoned a heavily trailed threat to prosecute militants for treason.
The Prime Minister announced last week that ministers were trying to win promises from around 10 countries that they would not torture deportees sent to them. But Mr Novak said that assurances from countries such as Jordan and Algeria that they they would not harm the suspects they received were worthless.
"If you deport people, whether they are British citizens or foreigners, to another country where they are subjected to the risk of torture, then this is absolutely prohibited under international human rights law," he told BBC Radio 4's Today programme. "If there is a substantial risk in a certain country like Algeria, Jordan, Egypt, etc, then diplomatic assurances cannot be used."
Mr Novak cited a recent example of a man deported from Sweden to Egypt, who was tortured in spite of promises by the Egyptian government that he would not be ill-treated.
"If a country usually and systematically practises torture, then of course they would deny they were doing it because it is absolutely prohibited so they can easily give diplomatic assurances but they are worth nothing."
Britain has concluded a memorandum of understanding with Jordan that suspects who are returned will not be harmed. Mr Blair has said Britain was "close to getting necessary assurances from other relevant countries", naming Algeria and Lebanon.
The Government has not named the other nations from which it hopes to win pledges not to torture deportees. They are thought to include Egypt, Tunisia, Morocco, Libya, Syria, Yemen and the United Arab Emirates. All retain the death penalty and, according to Amnesty International, there have been recent reports of torture in most.
Mr Bakri, who fled England for Beirut on Saturday, has joint Syrian-Lebanese nationality and so could be deported to either country under the Government's proposals.
Also likely to be among the first to be deported is Abu Qatada, a Jordanian who is now under house arrest after being released from Belmarsh prison.
Amnesty International recently highlighted reports that two Yemeni men were tortured over four days in Jordan. They claimed to have been severely beaten, being repeatedly hit on the soles of the feet with sticks while suspended upside down with hands and feet tied. It says torture is widespread in both Algeria and Lebanon.
Kate Allen, Amnesty's UK director, said: "The assurances of known torturers cannot be trusted and we have seen no indication of any monitoring to ensure that these promises are honoured." Announcing the moves, Mr Blair said France and Spain deported terrorist suspects to their home countries without breaching their human rights obligations.
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