The UK's anti-terror polices need to be "stiffened up," a senior Cabinet minister said.
Defence Secretary Michael Fallon said the country faced "very real threats" but denied that the Government's plans to prevent would-be jihadis from travelling to Iraq and Syria and keep out returning Britons considered a risk were a "knee-jerk response".
David Cameron will announce a new counter-terrorism measures in a Commons statement tomorrow.
The measures would make it easier to remove people's passports through temporary seizure powers at the border in order to prevent them travelling to the Middle East trouble zones.
Former Liberal Democrat leader Lord Ashdown hit out at the Tory response to the terror threat level, claiming the government had set out to frighten people in an effort to secure support for new powers.
But Mr Fallon told the Murnaghan programme on Sky News: "These are very real threats. We have had Tube trains blown up, London buses, Glasgow Airport attacked, a soldier murdered in broad daylight.
"This isn't any kind of knee-jerk response. The Prime Minister made clear on Friday we need to be calm and measured about the way that we do this."
The PM wants to confiscate passports. But in Cardiff they want to solve extremism at it's roots - they just can't get the help to do it.
David Cameron has unveiled a series of new measures to combat the threat of terrorism.
Rumours suggesting London's Tube network will be targeted in a terrorist attack on Monday are "completely untrue", police say.