A British aircraft took part in the air strike in Syria which led to the deaths of dozens of regime soldiers and has threatened to scupper the US-Russia deal against Isis.
It is believed that a UK Reaper drone was part of the American raid on Dayr az Zawr last Saturday.
A Ministry of Defence spokesman in London said “"We can confirm that the UK participated in the recent coalition air strike in Syria, south of Dayr az Zawr and we are fully cooperating with the coalition investigation. The UK would not intentionally target Syrian military units. It would not be appropriate to comment further at this stage.”