British security services have foiled around seven terror attacks since June with fighters returning from Syria posing a growing threat, Prime Minister David Cameron says.
"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.
"It was the sort of thing we were warned about."
Cameron also said 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.
"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.
Britain is to recruit an extra 1900 security and intelligence staff to counter the threat of terrorist violence following the Paris attacks, which killed at least 129 people, British media reported on Monday (local time).
It would be "the biggest increase in British security spending since the 7/7 bombings in London" that killed dozens in 2005. The measures will be announced by Cameron later on Monday, according to the Guardian.
"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 will say, according to the newspaper.
"This is a generational struggle that demands we provide more manpower to combat those who would destroy us and our values."
The recruitment would increase the staff of intelligence agencies MI5, MI6 and GCHQ by some 15 percent, according to the Guardian and the Financial Times.
In addition, extra aviation security officers would assess airports 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.