Counter-terror officials believe a serious, well-advanced Islamic extremist terror plot against the UK has been foiled after two alleged jihadists were arrested.

The plot is thought to have been inspired by the Islamic State group, with attacks planned in the capital London and the UK’s second city Birmingham.

The young suspects, aged 20 and 21, were held on Wednesday night “on suspicion of being involved in the commission, preparation and instigation of terrorism”, a statement from the Metropolitan force revealed.

Searches of addresses in Southeast Birmingham and North London were said to be ongoing and the Met’s Firearms Command was involved in the operation.

The arrests were linked to “UK-based activity”, counter-terror sources told The Telegraph, but the precise nature of the plot is still unknown as the investigation is in its early stages and suspects are yet to be interrogated.

23,000 Jihadists, the vast majority of whom MI5 and the Police don't have the time, money, or people to watch https://t.co/goUgrrIa73 — Breitbart London (@BreitbartLondon) May 27, 2017

The Islamic extremist threat to the UK is at an unprecedented scale and has increased “dramatically” over the past year, MI5 Director-General Andrew Parker revealed last month.

“In 2017, with all that has happened and much that has not, it is clear that we are contending with an intense UK terrorist threat from Islamist extremists,” he said in a rare public speech.

“That threat is multi-dimensional, evolving rapidly, and operating at a scale and pace we’ve not seen before… We’ve seen a dramatic upshift in the threat this year. It’s at the highest tempo I’ve seen in my 34-year career.

“Today there is more terrorist activity, coming at us more quickly, and it can be harder to detect,” he said.

Adding: “Islamist terrorism is an acute and enduring challenge that requires a sustained and comprehensive approach.”

There have been a number of successful Islamic fundamentalist attacks this year, including a van and knife strike at London Bridge, a suicide bombing at the Manchester Arena, and a partially-detonated bomb at Parsons Green Tube in the capital.