Paris-style attacks could "easily" happen in UK cities such as London, Manchester or Glasgow, British Defense Secretary Michael Fallon told Sunday Telegraph on Sunday.
Fallon stepped up efforts to win MPs' backing for air strikes against Daesh fighters in Syria, the BBC reported.
He said Daesh could only be beaten by force.
But he admitted it would be harder to get the support of the Commons if Labor leader Jeremy Corbyn ordered his MPs to vote against air strikes.
Fallon added he could not rule out attacks such as those in Paris - where 130 people were killed after gunmen and suicide bombers almost-simultaneously hit a concert hall, stadium, restaurants and bars.
"What happened in Paris and Brussels could easily happen in London," he said.
"The threat to the UK is extremely high. An attack is highly likely so we have to respond."
Britain has so far refrained from bombing Daesh in Syria after MPs rejected a vote for air strikes against Syrian government targets in 2013.
Fallon said he hoped the recent events would encourage a different outcome.
"I hope all MPs will weigh up the arguments. They need to keep us safe here in Britain, to respond to the United Nations resolution and to France's call for help.
"The RAF is making a difference in Iraq and we need to hit Isil (Daesh) harder and deal with this death cult once and for all.
"These aren't people you can negotiate with. You can only deal with them by force."
The defence secretary said he had informed the RAF not to presume that Parliament would support extending air strikes against Daesh targets into Syria as well as Iraq.
Source: MENA
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