The soldier, Ben Griffin, quit the elite Special Air Service (SAS) in protest at what he said were US abuses in Iraq, BBC's website reported.
He left the SAS in 2005 after taking part in operations in the Persian Gulf and said that British soldiers detained suspected extremists and interrogated them before handing them over to the US military.
"We were under no illusion as to what awaited the individuals handed over by us," he said in a statement issued by Stop the War Coalition, with which he is now involved. 
The revelation - which has raised questions that there may be more to follow - came after an administrative error on the part of the US, he added.
Meanwhile, Griffin says, "The UK government is deeply involved in the whole process of rendition." He has details of dates of operations in which he was involved and a letter from one British interrogator who was "upset at what he has been part of".