The threat to Britain from Isis is as “potent” as it was in Paris, where 130 people were massacred by extremists, the Defence Secretary has warned.
Mr Fallon said: “What happened in Paris can easily happen in London.
“The threat to the UK is extremely high. An attack is highly likely so we have to respond.”
He added that other major UK cities including Manchester and Glasgow could also be threatened by similar attacks.
Mr Fallon told The Sunday Telegraph that an attack similar to Paris’ and the threat to Brussels, which resulted in the city going into lockdown, cannot be ruled out in the UK, despite the UK’s security forces working hard to combat terrorist threats.
Opposing Mr Fallon, Labour leader Jeremy Corbyn has made it clear he cannot support RAF air strikes in Syria.
Mr Fallon said he respects the Labour leader’s views on air strikes, but argued force is necessary to defeat Isis. The only way to deal with the jihadists is by force, he added.
“These aren’t people you can negotiate with. You can only deal with them by force.”
Tom Watson, deputy Labour leader, has warned Mr Corbyn to back down over opposing UK air strikes on Isis in Syria.
Speaking to the Independent on Sunday, he said: “Matters of security are perhaps the most solemn decisions that MPs have to make and they have to think about it deeply.
“After the week we’ve had, the best way of holding the party together, but allowing MPs to solemnly express what they feel, is for us to have a free vote.”
Mr Fallon said the option of sending combat troops has been ruled out, and that any campaign against Isis would be a long one.
Britain has so far refrained from bombing Isis in Syria.
David Cameron said on Saturday he hoped MPs would do the right thing “when the choice comes”, indicating the Commons will be asked to decide on the issue.
