The film, which purports to show the killing of five men who IS says were spying for the UK, is from a group that is "losing territory" and "increasingly losing anybody's sympathy", he said.The UK's intelligence agencies are examining the footage.Voice-print analysis will be carried out to try to identify the masked man.
The man says the 10-minute video - which has not been independently verified - is a message for David Cameron. In it, he threatens attacks in the UK.
On a visit to east London, Mr Cameron said: "It's desperate stuff from an organisation that really does do the most utterly despicable and ghastly acts and people can see that again today."But this is an organisation that's losing territory, it's losing ground, it's, I think, increasingly losing anybody's sympathy, and this again shows what an appalling organisation we're up against.IS hated the UK for being a "successful, tolerant, democratic, multi-faith, multi-ethnic nation", he said.
"But I know that Britain will never be cowed by this sort of terrorism. Our values are so much stronger than theirs. It may take a very long time but they will be defeated," he said.