The Prime Minister knew that British military personnel embedded with US and Canadian air forces were involved in airstrikes over Syria, it has been revealed.
The Ministry of Defence has confirmed a handful of aircrew have effectively been operating as foreign troops, taking part in airstrikes against the so-called Islamic State in Syria.
David Cameron's spokeswoman said: "The Prime Minister was aware of UK personnel being embedded with US operations and what they were doing."
She said "upward of a dozen" Britons are embedded with other nations fighting the counter-IS campaign but currently none of them are pilots.
The news comes despite Parliament having only given authorisation to UK forces to attack IS targets in Iraq, where they are operating with the invitation of the Iraqi government.
In 2013, the Government lost a parliamentary vote after seeking backing for military action against Syrian president Bashar al Assad to deter the use of chemical weapons.
Sky News Defence Correspondent Alistair Bunkall said the UK pilots were flying in US Super Hornets off the USS Carl Vinson earlier this year.
He added: "On the face of it (this is) very surprising, given the very public humiliation the Government had a couple of years ago when it went to Parliament to try and have that approved and it wasn't approved by Parliament - therefore, the RAF was confined to Iraq."
He explained that the pilots - from the Royal Navy and RAF - had been flying embedded with the US and Canadian forces so were effectively not under British authority.
He said: "I don't think there is any attempt to try and flout British law to get around Parliament's restrictions on it.
"(But) when it was so publicly rejected by Parliament, they are being perhaps a little foolish to have British pilots putting themselves in that position."
American planes are carrying out around 92% of all strikes against Islamic State.
The UK effort amounts to about 4% of the total with the other coalition countries making up the rest.
The UK contribution includes eight Tornado jets, 10 unmanned Reaper drones, two Sentinel spy planes, 2 E-3D Sentry command and control aircraft, along with a Voyager refuelling aircraft, an Airseeker intelligence-gathering plane, Hercules transport aircraft and a Shadow special forces plane.
A Government spokesperson said: "The UK is contributing to the anti-ISIL coalition air campaign against ISIL targets in Syria through the provision of intelligence, surveillance and reconnaissance.
"The UK itself is not conducting air strikes in Syria. But we do have a long-standing embed programme with allies, where small numbers of UK personnel act under the command of host nations.
"That has been the case in Syria, although there are currently no pilots operating in this region."
Jennifer Gibson, staff attorney at Reprieve, the group who revealed the news from a Freedom of Information request, called for an "open and honest debate" about UK involvement in Iraq and Syria.
She added: "It is alarming that Parliament and the public have been kept in the dark about this for so long.
"Yet more worrying is the fact that the UK seems to have turned over its personnel to the US wholesale, without the slightest idea as to what they are actually doing, and whether it is legal."
Minister of State for Defence Procurement Philip Dunne said he has also been aware of the Syria strikes "for a period of weeks" but that Parliament did not need to be told UK pilots were flying strikes over Syria because they were under foreign command.
Labour leadership hopeful Andy Burnham told Sky News: "The will of Parliament must override and be upheld at all times."
He refused to rule out support for such involvement but added: "We have to proceed cautiously and responsibly, learning the lessons of the past where Britain has got involved perhaps without the right evidence and right justifications."
Liberal Democrat leader Tim Farron said the involvement of British pilots without Parliament's approval was " a breach of trust with the British people".
Tory backbencher John Baron, who opposed air strikes in Iraq, said ministers must come to the Commons to explain what had happened and that personnel should be pulled back from the embeds until after a parliamentary vote.
Defence Secretary Michael Fallon said on Thursday that the UK was engaged in a "new Battle of Britain" against IS, as he confirmed a second RAF Airseeker plane would be sent to the region.