British pilots have carried out military airstrikes in Syria, the Ministry of Defence has revealed.
The UK personnel were embedded with the forces of allied nations, including the USA and Canada, which have been conducting strikes against the Islamic State (IS) terror group.
The number of pilots involved is in single figures and none are currently taking part in airstrikes.
The House of Commons voted against military action in Syria in 2013 and parliamentary authorisation has so far been given only to UK air strikes against IS in neighbouring Iraq.
But the MoD said any personnel embedded in foreign nations' forces were effectively operating as troops of that country.
British troops are regularly embedded in other nations' forces and have operated under US command since the 1950s, including in recent operations in Afghanistan, Iraq and Libya. The MoD agrees each deployment and continually monitors the permissions granted to embedded troops.
UK forces are taking part in surveillance and air-to-air refuelling operations over Syria, and in the wake of the murder of 30 Britons in a terror attack in Tunisia last month, David Cameron and Defence Secretary Michael Fallon made clear they are considering extending airstrikes against IS into the country.
They have indicated that they would seek MPs' approval for an extension of airstrikes into Syria, but no vote is expected before the autumn.
Details of British personnel's involvement in strikes by allied nations' forces were revealed by a Freedom of Information request from pressure group Reprieve.
A Ministry of Defence spokesman said: "When embedded, UK personnel are effectively operating as foreign troops.
"The UK is contributing to the anti-Isil coalition air campaign against Isil targets in Syria through the provision of intelligence, surveillance and reconnaissance.
"Isil poses a direct threat to the UK and to countries around the world. The UK is not conducting air strikes in Syria.
"But we 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.
"When embedded, UK personnel are effectively operating as foreign troops."
Mr Fallon said on Thursday that the UK was engaged in a "new Battle of Britain" against IS, as he confirmed the deployment of a second RAF Rivet Joint spy plane to the region.
Earlier this month, he said there was an "illogicality" about targeting IS in Iraq but not Syria, when the terror group did not respect international borders.
He suggested that the new Parliament may take a different view from its predecessor on strikes in Syria. Downing Street has said that more "thought and deliberation" is needed before putting the issue to a vote in the Commons.