Two Royal Air Force jets are on their first combat mission over Iraq since the UK Parliament authorised air strikes targeting Islamic State (IS) militants.
The Tornado jets took off from the RAF Akrotiri base in Cyprus, the ministry of defence has confirmed.
It said the planes were "ready to use in an attack role as and when appropriate targets are identified".
The Tornado GR4 combat jets, armed with Paveway IV laser-guided bombs, took off this morning. In in an operation that began before dawn, the jets were loaded with Paveways before taking off from the Mediterranean island.
British lawmakers on Friday voted overwhelmingly to join the US-led air strikes in northern Iraq.
US bombing in Syria has disrupted the IS group's lucrative oil-pumping operations, the Pentagon has said.
Six Tornados have been based on Cyprus since last month. They have been conducting reconnaissance missions over Iraq, but their role will now change to striking IS targets following parliament's vote.
The vote followed a formal request for help from the Iraqi government.
"We can confirm that, following parliamentary approval given yesterday, RAF Tornados continue to fly over Iraq and are now ready to be used in an attack role as and when appropriate targets are identified," a defence spokesman said.
"For operational security reasons we will not be providing a running commentary on movements; we will provide an update on activity when it is appropriate to do so."
The House of Commons voted by a majority of 524 to 43 after the main political parties all approved military action.
IS's brutal abuses against civilians, rival fighters and Arab and Western hostages, as well as its success in recruiting Western members, have triggered international alarm.
British Prime Minister David Cameron said on Saturday the RAF input was one part of a large international coalition.
"But the crucial part of that coalition is that it is led by the Iraqi government, the legitimate government of Iraq, and its security forces. We are there to play our part and help deal with this appalling terrorist organisation."
BBC defence correspondent Jonathan Beale said it was possible the planes would return without having used their weapons, but that would be confirmed only later.
The Tornados, which were reported to have taken off at 08:30 BST, were supported by a Voyager air-to-air refuelling aircraft.