Foreign Secretary David Miliband has said he has "some concerns" over US military tribunals for six men charged with involvement in the 9/11 attacks.
Mr Miliband said the UK always asserted its system of values
The US government has promised fair trials for the Guantanamo Bay inmates, who could face the death penalty.
But human rights groups say the tribunals make this impossible and that the defendants were tortured.
Mr Miliband told BBC Radio 2's Jeremy Vine show there was "absolutely no question" that torture was illegal.
Alleged 9/11 plot mastermind Khalid Sheikh Mohammed is among those charged.
He and his co-defendants could face the death penalty if convicted of murder and conspiracy.
The BBC's Vincent Dowd in Washington says a confession gained from Khalid Sheikh Mohammed may prove problematic as the CIA admitted using "water-boarding" - or simulated drowning - as an interrogation technique.
In answer to a question from a Jeremy Vine show listener, Mr Miliband said the UK defined water-boarding as torture, adding that "we don't... we would never use water-boarding".
Mr Miliband said: "There's absolutely no question about the UK government's commitments in respect of torture, which is illegal, and our definition of what torture is.
"And I think it's very, very important that we always assert that our system of values is different from those who attacked the US and killed British citizens on 11 September, and that's something we'd always want to stand up for."
Any trials would be held by military tribunal under the terms of the Military Commissions Act, passed by the US Congress in 2006.
The act set up tribunals to try terror suspects who were not US citizens.
But human rights groups have questioned whether these can be fair.
'Check and balance'
Asked if the trial of Khalid Sheikh Mohammed would respect his legal rights, Mr Miliband said: "We have some concerns about that, and there are still some cases in front of the American Supreme Court, because, of course, the great thing about America, and I suppose countries like ours as well, is that the independent legal system provides a check and a balance on the operation of the legal system itself.
"And so the Supreme Court has already ruled against some of the tribunals that have been established from the Guantanamo experience, and there are some cases in respect of what's called the Military Commissions Act, which is the basis on which he'd be tried, that are being discussed in front of the Supreme Court at the moment."
The charges are the first for Guantanamo Bay inmates directly related to the 9/11 attacks.
The detention centre, in south-east Cuba, began to receive US military prisoners in January 2002.
Hundreds have been released without charge but about 275 remain and the US hopes to try about 80.
Nineteen men hijacked four planes in the 9/11 attacks. Two planes hit the World Trade Center in New York, another the Pentagon in Washington and the fourth crashed in Pennsylvania.