The prominent human rights group Liberty demanded Wednesday the foreign secretary and eight domestic police forces investigate the allegations within 14 days. If they do not comply, the group will launch a court action to "enforce their obligation" to investigate, Liberty Director Shami Chakrabarti told United Press International.
Liberty said the British government would be in breach of international and domestic law if it was found to be allowing U.S. "extraordinary rendition" flights to stop and refuel at its airports.
International outrage has been mounting since The Washington Post reported earlier this month the CIA was operating a global network of so-called "black sites," secret prisons where it holds prisoners incommunicado. There, it is alleged, they are tortured.
Several human rights groups, including the New York-based Human Rights Watch and Amnesty International, say they have evidence from CIA flight logs corroborating both these allegations and claims the United States is "franchising" out interrogation of detainees to regimes that practice torture.
The logs also suggest flights passed through several European countries, they say, including Britain. Spain, Sweden, Iceland and Germany are all investigating separate reports; Britain, however, has taken no action.
Chakrabarti acknowledged that much of the evidence was circumstantial: Flight patterns, for example, but stressed: "All we are saying is that this warrants an investigation.
"It's not an awful lot to ask, we say, of a government that cares about human rights."
Britain had a "positive obligation" under human rights law to do so, she added.
Allowing ourselves to become complicit in secret detentions and torture would "debase our humanity" and "hand an enormous propaganda victory to the terrorists," she told UPI.
Labor Member of Parliament Chris Mullin, one of several MPs pressing the government on the claims, said he was not aware of any evidence that it was "complicit" in the practice.
But, he said, Britain had shown a "lack of curiosity" about it.
He told the BBC: "There's no doubt some sort of secret gulag exists which is controlled by the Americans into which people disappear for months at a time.
"And there's also no doubt that the Americans have for some time been franchising out torture to countries that are rather less scrupulous than ourselves, and indeed the Americans, about the use of torture."
Britain has, as the current European Union president, written to the United States to inquire about the alleged "black sites" and use of EU airports for rendition.
Foreign Office Spokeswoman Kay Stokoe told UPI the inquiry included the alleged use of British airports. The government was not aware of any evidence corroborating the allegations, she stressed.
The Foreign Office was considering the contents of Liberty's letter and could not comment further, Stokoe added.
In the past, the government appears to have turned a blind eye to flights passing through British airports.
A well-placed diplomatic source told UPI it was "basically a matter for the Americans," while Armed Forces Minister Adam Ingram told Parliament last week that officials did not inquire as to the identity of passengers when U.S. aircrafts passed through military airfields.
His answer was given to Liberal Democrat Foreign Affairs Spokesman Sir Menzies Campbell, who is heading the group of influential MPs pressing the government.
Menzies Campbell said Wednesday: "We need full disclosure by the government.
"If in fact people are being moved from a jurisdiction where torture is illegal to a jurisdiction where torture is permissible, that seems to me to be wholly contrary to international law.
"If we are allowing facilities for aircraft carrying out those actions, then we are at the very least facilitating it, we may even be complicit in it."
The United Nations is also investigating the allegations, after Britain's Guardian newspaper claimed in September to have evidence that aircraft used for rendition had landed in Britain at least 210 times since the 9-11 attacks.
It is alleged at least 12 airports in England and Scotland have been used, with the busiest being Prestwick in Ayrshire, where CIA aircraft landed more than 75 times.
The EU is investigating claims of clandestine CIA jails in eastern European countries. Human Rights Watch says the prisons are situated in Romania and Poland. Both countries have denied any knowledge of the sites.
A BBC "Newsnight" investigation reported Tuesday found that in 2003, two planes regularly used by the CIA -- a Gulfstream 5 and a Boeing 737 -- landed in Poland at least five times. Three of the flights came via Kabul, Afghanistan.
It detailed one itinerary of a Boeing 737 flight through Poland. Departing from Washington on Sept. 20, it flew to Tashkent in Uzbekistan via Prague in the Czech Republic. On Sept. 21 it landed in Kabul, Afghanistan, before continuing to Szczytno, Poland. It then flew to Constanta, Romania, and on to Rabat, Morocco. On Sept. 22 it made its final stop at Guantanamo Bay, Cuba.
Stephen Crawshaw, of Human Rights Watch UK, said: "The question then comes up, why would you be stopping off in a remote airfield in, say, Poland, or, say, Romania? It's not a refueling stop, so what is it doing there?" he said.
Meanwhile, the European Council has appointed Swiss Senator Dick Marty to investigate what he called the suspicious movement patterns of flights through Europe.
The EU's top justice official, Franco Frattini, said earlier this week that he would recommend the suspension of voting rights for EU countries found to have breached the bloc's founding principles of human rights, democracy and the rule of law.