British prime minister David Cameron was aware that RAF pilots were taking part in bombing raids over Syria despite MPs having voted against Britain carrying out strikes in the country.
Downing Street said the prime minister knew that a small number of aircrew were embedded with US and Canadian forces and “what they were doing”.
The news came amid growing pressure for a statement to parliament after the activity emerged in response to a freedom of information request.
Currently, parliament has only authorised UK forces to attack Islamic State (IS) targets in neighbouring Iraq, where they are operating at the invitation of the government in Baghdad.
Mr Cameron’s spokeswoman told journalists: “The PM was aware that UK personnel were involved in US operations and what they were doing”.
She said the policy of embedding UK personnel with foreign forces had been in place since the 1950s and was “well known”. The ministry of defence had been asked about the issue before, she insisted.
She said “upward of a dozen” Britons were embedded with other nations engaged in the counter-IS campaign – although currently none of them are pilots.
Conservative backbencher John Baron, who opposed air strikes in Iraq, said ministers must come to the Commons to explain what had happened.
"We had a major vote. There should be sensitivity on this issue, and we should be very sensitive to the fact that we have military personnel participating, in effect, in military intervention," he told the BBC Radio 4 Today programme. – (PA)