David Cameron has described a murderous Islamic State video featuring a masked extremist and a young boy with British accents as "desperate stuff" from a group that is "losing territory" and "increasingly losing anybody's sympathy".
The Prime Minister said "Britain will not be cowed" by the terror group, which has released footage online appearing to show the brutal killing of five men accused of spying for the UK.
A jihadi is seen waving a gun as he directly mocks Mr Cameron and the RAF bombing campaign against IS in Iraq and Syria.
The Prime Minister’s official spokeswoman also confirmed RAF pilots have carried out 11 airstrikes in Syria since MPs voted in support of bombing last month - including targeting checkpoints on Christmas Day.
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.
"They hate us not for what we do but for what we are - the fact that we are a successful, tolerant, democratic, multi-faith, multi-ethnic nation.
"They hate that and that's why they want to take us on and that's why they do what they do.
"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."
Earlier in the day, Downing Street said the “barbaric” new ISIL execution video is a “propaganda tool” that shows the terrorist group is “under pressure”.
At the daily briefing of Lobby journalists at Westminster, David Cameron’s spokeswoman said spooks are still examining footage of the masked militant.
David Cameron is threatened in the ten-minute clip, and mocked as an “imbecile” boasting a just a “handful of planes” carrying out bombing raids in Iraq and Syria.
The video, thought to be shot in Raqqa in Syria, ends with a child believed to be from the UK hailing the deaths.
Today, the PM’s spokeswoman said: “We are examining the content of the video and the PM is being kept updated on that. It serves as a reminder of the barbarity of Daesh and what the world faces with these terrorists.
“It’s also clearly a propaganda tool and should be treated as such. This is a terrorist group being put under pressure with targeted strikes that have successfully targeted some of the key proponents in the organisation with the action the Iraqi forces are being able to take to push back against Isil.”
She was also questioned about the extent of RAF attacks in Syria after MPs sanctioned bombings last month. The Sunday Telegraph reported that there has been three missions - but none since December 6.
She confirmed there have been 11 missions inside Syria since the vote, as well as in Iraq, which had been given the green light by Westminster a year ago.
“We have conducted more airstrikes against Daesh inside Syria, including targeting checkpoints south of Raqqa on Christmas Day,” the spokeswoman said.