The security services successfully foiled more than 12 UK terror attacks last year, Defence Secretary Sir Michael Fallon has revealed.

In the aftermath of the Westminster attack, however, the Defence Secretary also admitted that Britain now faced a new type of lone-wolf, low-tech terror threat that was “much more difficult” to prevent.

Speaking the day after a terrorist used a car to mow down pedestrians on Westminster Bridge before stabbing a policeman outside Parliament, Sir Michael told BBC Radio 4’s Today programme: “This kind of attack, this lone wolf attack, using things from daily life – a vehicle, a knife – is much more difficult to forestall.

“We are also dealing with a terrorist enemy that is not making demands or holding people hostage, but simply wants to kill as many people as possible – so this is a new element to international terrorism.

“This is a new type of attack where somebody gets hold of a vehicle and drives it into innocent people – that also shows the moral vacuity of these terrorists.”

But he insisted that the security services were successfully foiling terror plots, saying: “The police and agencies that we rely on for our security have forestalled a large number of attacks in recent years – over a dozen last year.”

This seems a significant increase on some previously disclosed figures.

In October, Neil Basu, deputy assistant commissioner of the Metropolitan police and the senior national co-ordinator for counter-terrorism policing, said the security services had “successfully foiled at least 10 attacks in the last two years.”

In pictures: Westminster attack Show all 9 1 /9 In pictures: Westminster attack In pictures: Westminster attack An air ambulance lands after gunfire sounds were heard close to the Palace of Westminster in London PA wire In pictures: Westminster attack MPs wait until the situation is under control in Westminster. 'The alleged assailant was shot by armed police,' David Lidington, leader of the House of Commons, told the house. BBC News In pictures: Westminster attack Crowds gather in Westminster after shooting incident, which police are treating as terror attack BBC News In pictures: Westminster attack Police were also called to an incident on Westminster Bridge nearby AP In pictures: Westminster attack Early reports indicate the car, which mounted the pavement on Westminster Bridge and mowed into around a dozen people, was the same vehicle which then rammed into the railings of the Palace of Westminster, just around the corner Reuters In pictures: Westminster attack Security sources described the suspected assailant as a middle-aged Asian man, who is understood to have left the car before attacking a police officer with a seven-to-eight inch knife PA wire In pictures: Westminster attack Police have asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident AP In pictures: Westminster attack One woman has died and a number of others, including the police officer, have been hurt, according to a junior doctor at St Thomas' Hospital Reuters In pictures: Westminster attack At least three gun shots were heard by those inside Westminster, and proceedings in the House of Commons have been suspended AP

Suggesting the scale of the threat faced by the UK, Mr Basu added: “We continue to work at a relentless pace with our key partners, currently dealing with around 550 live cases at any one time.”

One attempted plot, it was revealed last August, had been thwarted “in the final hours before the planned attack” on a UK target.

An operation run by GCHQ, the British intelligence and security organisation, used interception warrants to monitor phones owned by individuals linked to the 2015 plot, it was reported.

According to the Independent Reviewer of Terrorism Legislation, agents were then able to identify and intercept a would-be terrorist cell that was close to carrying out its plan.

Answering questions about whether the security services had enough resources to deal with such levels of threat, Sir Michael Fallon revealed this had been “reviewed” at an emergency COBRA meeting chaired by Prime Minister Theresa May on Wednesday night.

He said: “We reviewed whether the Met had all the resources they need – including military back up – to deal with situations like this.

“That is something we always review at the time.