The Ministry of Defence said 1,571 IS fighters had been killed in Iraq since bombing began two years ago, and 181 had died in Syria since December 2015. It said "detailed assessments" showed that no civilians had been killed.
Monitoring group Airwars said if the claim was true, it was "unprecedented in the history of modern warfare".
Airwars said this was only an "air-only" assessment which meant the UK could not claim there were no civilian deaths. Airwars director Chris Woods said he had raised concerns about 35 incidents in the first six months of this year where there had been reports of civilian deaths.
"Britain is in the uncomfortable position of being in the same position as Russia in claiming that large numbers of air strikes have killed no civilians," he said. "From the air alone they cannot know the effect that their air strikes are having on the ground."