The resurgence of ISIS and Al-Qaeda is the terrorism "trend that concerns me most", the head of MI5 has admitted.

The Security Service’s Director General, Ken McCallum, warned of a "worsening threat" after a "few years of being pinned well back" by military operations in the Middle East.

He warned “organised groups have the numbers and the know-how to carry out, or inspire, horrendous mass casualty attacks”.

More than a third of MI5’s “top priority investigations” last month had an overseas link, Mr McCallum said.

Overall, MI5 and the police have disrupted 43 late-stage attack plots since March 2017 saving "numerous lives" he said, adding: "Some of those plotters were trying to get hold of firearms and explosives, in the final days of planning mass murder."

And Britain’s top intelligence officer pointed to the ISKP atrocities in Moscow, where 145 people were slaughtered, and the Kerman bombings in Iran, where 103 people were killed, as a “brutal demonstration of its capability”.

And more Britons are travelling to join terrorist groups around the World, Mr McCallum told the Daily Express. This includes ISKP in Afghanistan.

Outlining the threats facing Britain, Mr McCallum said: “We are powerfully alive to the risk that events in the Middle East directly trigger terrorist action in the UK, as we saw with last October’s knife attack in Hartlepool.

“The ripples from conflict in that region will not necessarily arrive at our shores in a straightforward fashion; they will be filtered through the lens of online media and mixed with existing views and grievances in unpredictable ways.

“I’ll finish here with the terrorist trend that concerns me most: the worsening threat from Al-Qaeda and in particular from Islamic State.

“Today’s Islamic State is not the force it was a decade ago. But after a few years of being pinned well back, they’ve resumed efforts to export terrorism. The ISKP attack in Moscow was a brutal demonstration of its capability.

“We and many European partners are detecting IS-connected activity in our homelands, which we are moving early to disrupt.

“And Al-Qaeda has sought to capitalise on conflict in the Middle East, calling for violent action.

“To illustrate, over the last month, more than a third of our top priority investigations have had some form of connection, of varying strengths, to organised overseas terrorist groups.

“Organised groups have the numbers and the know-how to carry out, or inspire, horrendous mass casualty attacks.”

At least 43 terrorist plots have been disrupted since March 2017, including conspiracies to set off bombs and shoot innocent people.

Mr McCallum warned online propaganda linked to the war in the Middle East and brutal images of the conflict could radicalise more people.

More than one in ten of those being investigated by MI5 for terrorism offences are under the age of 18.

“As events unfold in the Middle East, we will give our fullest attention to the risk of an increase in – or broadening of – Iranian state aggression in the UK.

“Like the Russian services, Iranian state actors make extensive use of criminals as proxies – from international drug traffickers to low-level crooks.

“So, to those tempted to carry out such tasks, I say this: If you take money from Iran, Russia or any other state to carry out illegal acts in the UK, you will bring the full weight of the national security apparatus down on you.

“It’s a choice you’ll regret.”

Mr McCallum has also revealed 20 Iranian assassination plots have been disrupted over the past two years.

He warned “we’ve seen plot after plot here in the UK, at an unprecedented scale”.

The Director General added: “As events unfold in the Middle East, we will give our fullest attention to the risk of an increase in – or broadening of – Iranian state aggression in the UK.

“Like the Russian services, Iranian state actors make extensive use of criminals as proxies – from international drug traffickers to low-level crooks.

“So, to those tempted to carry out such tasks, I say this: If you take money from Iran, Russia or any other state to carry out illegal acts in the UK, you will bring the full weight of the national security apparatus down on you.

“It’s a choice you’ll regret.”