Defence Secretary Michael Fallon is facing calls for a Commons statement following the disclosure RAF pilots have taken part in air strikes on Syria.
The Ministry of Defence said the small number of aircrew involved had been embedded with US and Canadian forces attacking Islamic State (IS) forces and were effectively operating as foreign troops.
However MPs from across the political spectrum expressed concern that British personnel were involved after the House of Commons expressly voted against air strikes on Syria two years ago.
Currently, Parliament has only authorised UK forces to attack IS targets in neighbouring Iraq, where they are operating at the invitation of the government in Baghdad.
Tory backbencher John Baron, who opposed air strikes in Iraq, said that 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.
"Those individuals should be withdrawn from the embedded programme whilst this vote holds sway, while it still has authority, until we vote again.
"This is, at the end of the day, what parliamentary democracy is all about, regardless of the pros and cons of military intervention."
New Liberal Democrat leader Tim Farron said the involvement of RAF pilots in air strikes without the approval of Parliament was "a breach of trust with the British people" that would simply play into the hands of IS - also referred as Isis or Isil.
"They desperately want the West to attack them and to be seen to attack them. We are utterly playing in to their hands if we do this," Mr Farron told Sky News.
"For us to be involved in this at this stage, without the sanction of the British people through Parliament and without proper thought being given to the way in which we almost strengthen Isis by doing this, seems very wrong."
Labour, which recently indicated it could be prepared to back a fresh vote on extending air strikes into Syria, said that it would be calling on ministers to make a Commons statement on Monday about the role British pilots had played.
"Ministers need to be clear about the extent of their involvement and as clear as possible about any action they were involved in," a spokesman said.
"The programme to embed personnel with allies is extremely valuable to the development of our armed forces but the Government needs to be transparent about the role they play."