Mr Mohamed has said he was tortured while he was held in US custody
Britain has been accused of taking part in US "renditions" of terror suspects in a United Nations report.
UN Special Rapporteur Martin Scheinin said he was "deeply troubled" at the US system of rendition, secret detention and practices violating torture bans.
But he said it was only possible with the help of US allies including the UK.
The Foreign Office said in reply that Britain 'abhorred' torture and condemned any 'extraordinary rendition' transporting people to be tortured.
In the report, Mr Scheinin says the US system of rendition - in which suspects were seized, then transferred to covert CIA detention centres known as "black sites" in countries like Afghanistan and Morocco - required "an international web of exchange of information".
He said it had "created a corrupted body of information which was shared systematically with partners in the war on terror through intelligence co-operation, thereby corrupting the institutional culture of the legal and institutional systems of recipient states."
Among those named as having helped the rendition process - either through providing intelligence or seizing suspects - were the UK, Bosnia and Herzegovina, Canada, Croatia, Georgia, Indonesia, Kenya, Macedonia and Pakistan.
The rapporteur went on to say that countries are "responsible" if they knowingly help other states carry out human rights violations and should limit co-operation with such countries.
He also drew attention to reports of British interrogations of suspects held by the US in Pakistan, Afghanistan, Guantanamo Bay and Iraq.
And he named Britain among countries which had provided intelligence on individuals or "conducted the initial seizure".
He said he was "worried" about the UK's use of "state secrecy provisions" to hide "illegal acts from oversight bodies or judicial authorities, or to protect itself from criticism, embarrassment and, most importantly, liability".
In reply to the allegations, the Foreign Office said: "There is nothing new in these claims.
"The UK's position on torture is clear. We abhor torture. We don't participate in, solicit, encourage or condone it.
"We unreservedly condemn any practice of extraordinary rendition to torture."
MPs have demanded a judicial inquiry into claims that MI5 was complicit in the torture of a British resident in Morocco.
Binyam Mohamed, who returned to Britain after four years in Guantanamo Bay last month, claims MI5 fed his captors questions, at a time he said he was being tortured in Morocco. The government says it never condones torture and says Mr Mohamed's case has been referred to the attorney general.
Responding to the release of the UN report, Clive Stafford Smith - director of Reprieve, the legal charity that represented him - said the UN report was "rather familiar" to anyone who knew Mr Mohamed's story.
"The government must take this international criticism to heart and, instead of merely repeating platitudes about being opposed to torture, must show that when a British agent discovers some victim in some foreign torture chamber, Britain will take action to stop the abuse."
Lib Dem spokesman Edward Davey said: "It is shameful that we now seem to be reliant on outside organisations to uphold the rule of law in our own country."
He said the "continuing delay" in Mr Mohamed's case was unacceptable and a decision on whether there should be a police inquiry should be taken as soon as possible.
"It is a dark day for the reputation of Britain's secret services when a UN Special Rapporteur lumps them alongside those of Pakistan and Indonesia for co-operating with illegal activities linked to abduction and torture."
For the Conservatives, Pauline Neville-Jones - a former head of the Joint Intelligence Committee - said the government had to answer "the serious claims made about torture and rendition".
"Constant allegations which are not answered are damaging the good name of this country and undermining the credibility of the government's position that it neither practises nor condones torture," she said.