British security services have foiled around seven terror plots since June with fighters returning from Syria posing a growing threat, Prime Minister David Cameron said Monday.
"Our security and intelligence services have stopped something like seven attacks in the last six months, albeit attacks planned on a smaller scale" than Friday's attacks in Paris, he told BBC Radio 4 from Turkey.
"We have been aware of these cells operating in Syria that are radicalising people in our own countries, potentially sending people back to carry out attacks," he added.
Security services have spent a "long time" working out how to deal with multiple coordinated attacks on the street, but will have to go "right back to the drawing board" after the Paris attacks, which killed at least 129 people.
"It was the sort of thing we were warned about," said the prime minister.
Cameron added there were "hopeful signs" from Saturday's talks in Vienna on Syria that progress was being made on how to deal with the Islamic State (IS) group.
"You can't deal with so-called Islamic State unless you get a political settlement in Syria that enables you then to permanently degrade and destroy that organisation," he said.
However, he repeated that any settlement must include the removal of Syrian President Bashar al-Assad, a sticking point between the West and Russia.
- 'Generational struggle' -
Britain announced Monday it was to recruit an extra 1,900 security and intelligence staff to counter the threat of terrorist violence following the Paris attacks as part of the government's five year defence and security review to be unveiled next week.
It will be "the biggest increase in British security spending since the 7/7 bombings in London" that killed dozens in 2005.
"I am determined to prioritise the resources we need to combat the terrorist threat because protecting the British people is my number one duty as prime minister," Cameron said Monday.
"This is a generational struggle that demands we provide more manpower to combat those who would destroy us and our values," he added.
The recruitment would increase the staff of intelligence agencies MI5, MI6 and GCHQ by some 15 percent.
In addition, there will be a doubling of British funding for aviation security around the world, in response to the crash of a Russian plane in Egypt last month that the British government suspects may have been downed by a bomb.
British media reported the current funding for aviation security at around £9.0 million (12.7 million euros, $13.7 million).
"The Prime Minister has ordered a rapid review of security at a number of airports around the world in the wake of the Sinai disaster," said the government statement released by Cameron's Downing Street office.
"Additional security measures were put in place by the UK and US at a number of potentially vulnerable airports over the past year, and these will now be reviewed to check whether they go far enough."
Britain is engaged in air strikes against IS in Iraq, but has not joined in the operation over Syria.
Cameron wants to extend the mission, but said first he needs to "build the argument and convince more people."
When asked what ordinary citizens could do in face of the terror threat, the prime minister urged them to "be vigilant" but "remember our freedom depends on resolve and carrying on with our way of life." afp