The british jihadis filmed themselves warning of attacks in London Memri TV

Two Islamic State (Isis) supporters have claimed that Downing Street, Gatwick and Heathrow "are next" after the Brussels terror attacks. In a chilling video, the men wearing black masks and speaking with strong London accents, are seen celebrating the Brussels bombings and mocking the plight of the people of Belgium, before posting their message on Instagram.

One of the men says: "Belgium, Brussels airport big boom went off...innit. Heathrow..." The second jihadi adds: "Gatwick, Downing Street... you get me bruv..??'

Security experts believe the men are both ISIS fighters based in Syria where they used an internet cafe to film their threats. It is feared they were trained by Birmingham cyber-hacker Junaid Hussain, who was believed to have helped recruit and train many British-born jihadis. Hussaino was killed in a U.S. drone strike in August last year.

British security agencies estimate there are at least 400 British extremists based in Syria and around the same number have returned to the UK from the fighting. Earlier this month leaked Isis membership files exposed a number of British jihadists, recruiters and fighters in Syria and Iraq, who were previously unknown to the UK public. The newly revealed jihadists include fighters from Arsenal and Finsbury Park in north London.

Yesterday it was reported that Daesh (Isis)supporters have been celebrating the horrific terror attacks in Brussels by distributing sweets to residents in Syria and launching a poll asking for suggestions for the next city to target.

In a forum page, mocking the Paris tribute to the 31 people killed in the barbaric attacks on Brussels, one user set up the question poll asking: "What will be the color of the Eiffel Tower in the next attacks?" The United Kingdom came out top on the poll for ISIS's next target with half of the supporters on the forum backing it as the prime target for a terror attack.

Heathrow Airport
Heathrow airport has been marked as a potential target by Isis militants in a chilling video Getty

IS supporters posted threats on Twitter, tweeting: "The Islamic State will attack London, Washington, Rome and all the infidels' capitals." Another wrote: "The Crusaders are blundered and confused. Yesterday Paris, today Brussels, and they don't know where will be the next attack."

The latest threats come as security has been "stepped up" at checkpoints staffed by British officials on borders. However the increase in security will last for only two weeks.

Lucy Moreton, general secretary of ISU, said: "We've stepped up controls at the border, and at all borders, but it is maintaining that at a high level for a long period of time and whether there is the political will, or potentially even the necessity to do so. Experience from past would suggest that it isn't. The increased checks at the border last about two weeks, or that's how long they lasted after the Paris attacks."

Home Secretary Theresa May revealed yesterday in the House of Commons that intelligence services had foiled seven terror plots on British soil in the last 18 months.

The video, which emerged as police reported a significant arrest in terror raids in Paris, has been picked up by the Washington based Middle East Media Research Institute (MEMRI) which monitors extremist propaganda.