An inquiry has found no evidence that British airports were used by the CIA flying terrorist suspects for torture in other countries.
The Council of Europe has accused the UK of collusion
The investigation by the Association of Chief Police Officers followed claims by campaign group Liberty concerning "extraordinary rendition" flights.
But aviation expert Chris Yates said he believed evidence to prove the planes came to the UK was "out there".
He said the government had supported rendition flights "by any definition".
Liberty insisted that its claims were based on "credible investigations".
It had alleged that 210 CIA flights carrying terrorist suspects for possible torture had entered Britain since 2001.
Earlier a report by the Council of Europe alleged flights did pass through the UK and other European countries.
Acpo's findings emerged hours after the Council of Europe had also said there was evidence to prove the CIA had run secret jails in Europe.
It said prisons were used to interrogate terrorism suspects "in Europe from 2003 to 2005, in particular in Poland and Romania".
A year ago the report's author, Swiss MP Dick Marty, named the UK as one of 14 European countries which he said had colluded with the CIA in the operation of secret flights delivering terrorist suspects for interrogation.
The CIA later dismissed that report, with a spokesman saying it was biased and distorted, and that the agency had operated lawfully.
The chief constable of Greater Manchester, Michael Todd, began an investigation into Liberty's claims.
In a letter sent by Liberty to ACPO in November 2005, the group had claimed there was a "very real suspicion that the CIA is transporting people against their will through this country on their way to deliver them to state authorities in third countries" where they would be subjected to "torture at the hands of those authorities".
If this was the case, it said, then the CIA operatives were committing various offences while on British soil - torture, aiding and abetting and/or conspiracy to torture, false imprisonment and kidnapping.
A statement released by Acpo following the inquiry said: "The issue of rendition has been aired extensively in the media and has featured prominently in official reports over a recent period of months.
"Mr Todd has now examined all of the information available relating to this issue and has concluded that there is indeed no evidence to substantiate Liberty's allegations.
"There was no evidence that UK airports were used to transport people by the CIA for torture in other countries."
But Liberty's Shami Chakrabarti questioned how closely Mr Todd had looked into the allegations.
She insisted Liberty's complaint had been based upon "credible investigations that Britain had been used as a staging post".
Ms Chakrabarti said Liberty had plenty of evidence of CIA activity in terms of aircraft coming through the UK and testimony from people who say they had been rendered, but did not have the police's investigating powers to prove particular cases.
"When politicians spin it is disappointing. When police engage in the same activity it is rather more dangerous," she said earlier.
Mr Yates, who carried out an investigation into rendition flights in 2005, said there was "absolutely no doubt whatsoever" that the planes have visited the UK.
"There's enough evidence in terms of photographs and logs of planes taking off and comparisons of aircraft tail numbers," he said.
"These flights tally with certain activities which took place on certain dates in other countries - the picking up of CIA suspects.
Chief Constable Michael Todd responded to Liberty's complaint
"By any definition the government has given support to the CIA in rendition flights, either by clearance through UK airspace or clearance to use our airports.
"The question as to whether any of the rendition passengers were on the planes or whether they were normal CIA flights, which do visit this country, we will never know. It's all smoke and mirrors."
Liberal Democrat foreign affairs spokesman Michael Moore said that the inquiry does not "let the government off the hook".
He said: "The government has persistently failed to ask the right questions of the US authorities and so it is hardly a surprise that there's little new material on which the police can comment."
It was alleged in 2005 that airports at Biggin Hill, Birmingham, Bournemouth, Gatwick, Heathrow, Luton and Stansted, and RAF bases at Brize Norton, Mildenhall, Farnborough and Northolt had allowed CIA or CIA-chartered jets to land.
Liberty raised the issue in letters to the chief constables of Bedfordshire, Dorset, Essex, Hampshire, the Metropolitan Police, the Ministry of Defence Police, Suffolk, Sussex, Thames Valley and West Midlands.
The Foreign Office previously said the US had requested permission to transfer detainees through the UK on four occasions - all of them in 1998.
The government had granted two of the requests, and the individuals had stood trial in the US, according to the Foreign Office.