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Now British ISIS fighters threaten Heathrow and Downing Street in chilling video celebrating Brussels terror attacks

By Robert Verkaik for MailOnline10:47 24 Mar 2016, updated 15:23 24 Mar 2016

Masked men recorded laughing about the terror attacks which killed 34
They warn Downing Street, and Gatwick and Heathrow airports will be next
The men, none of whom can be identified, speak with British street accents
For more of the latest Islamic State news visit www.dailymail.co.uk/isis

Two British Islamic State fighters have threatened terror attacks on Heathrow, Gatwick and Downing Street in a chilling video celebrating theBrussels bombings.

The men, wearing black masks and speaking with London accents, filmed themselves laughing at the Belgium atrocity before posting their message on Instagram.

One of the men proclaims: 'Belgium, Brussels airport big boom went off...innit. Heathrow...'

They are feared to have been trained by Birmingham cyber-hacker Junaid Hussain, a major recruiter for ISIS.

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The video showed the masked man speaking towards the camera while wearing a balaclava to mask his face

Another jihadi with a British accent joins in laughing: 'Gatwick, Downing Street... you get me bruv..??'

The video has been picked up by the Washington based Middle East Media Research Institute (MEMRI) which monitors extremist propaganda.

Security experts believe the men are both ISIS fighters based in Syria where they used an internet cafe to film their threats.

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The menacing video is further evidence of the determined threat facing the UK from British jihadists who have gone to fight in Syria.

British security agencies estimate there are at least 400 British extremists based in Syria and around the same number who have returned to the UK from the fighting.

Although the identities of the men in the video are not known, it is possible they were trained by Birmingham cyber-hacker Junaid Hussain.

Hussain, who was killed in a U.S. drone strike in August last year, was believed to have helped recruit and train many British-born jihadis.

Among the Brits known to be fighting for ISIS is Siddhartha Dhar, a former bouncy castle salesman from London.

He fled to the militants' territory in 2014 after he was bailed on suspicion of encouraging terrorism.

Also known as Abu Rumsayah, it is believed that he is the masked executioner, dubbed Jihadi Sid, who featured in a graphic new ISIS video earlier this year.

The sickening video featuring Dhar also portrayed four-year-old Isa Dare, the young son of Grace 'Khadija' Dare, 24, a British woman who had links to the killers of Lee Rigby.

He was brainwashed by ISIS fanatics after being taken to Syria three years ago by Grace, who is married to the ISIS militant Abu Bakr.

Yesterday it was reported ISIS supporters have been celebrating the horrific terror attacks in Brussels by launching a poll asking for suggestions for the next city to target.

Birmingham cyber-hacker Junaid Hussain is feared to have trained the British jihadis who posted a video glorifying the Brussels attacks. He was killed in a US drone attack against Islamic State in Syria last August

Among the Brits known to be fighting for ISIS is Siddhartha Dhar (pictured), a former bouncy castle salesman from London

The masked executioner nicknamed Jihadi Sid

Isa Dare (pictured), 4, the son of Muslim convert Grace 'Khadija' Dare from south east London was seen pushing a button which detonated explosives in a recent ISIS propaganda video

He and others heard in the background can be heard saying Downing Street, Heathrow and Gatwick airports will be the next targets of terror

The British jihadis who feature in the video praising the Brussels attacks threatened Downing Street (pictured)

On a forum page, one user set up the question poll: 'What will be the color of the Eiffel Tower in the next attacks?'

The question comes after the iconic French structure was lit up in the colours of the Belgian flag, in solidarity to the victims of the Brussels 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 their top choice for a terror attack.

This month leaked Islamic State membership files exposed a number of British jihadists, recruiters and fighters in Syria and Iraq previously unknown to the UK public.

The newly revealed jihadists include fighters from Arsenal and Finsbury Park in north London.

Fourteen Islamic State recruitment forms were passed to the UK media by German newspaper Süddeutsche Zeitung. The forms include names, dates of birth, home towns and recruiter details.

There are fears that the men in the video were trained by Hussain, who was killed in a US drone attack against Islamic State in Syria last August.

He had tweeted under the alias Abu Hussain al-Britani and fled the UK two years ago while on police bail.

Now experts from the US-based Counter Extremism Project (CEP) say his ‘cyber warfare’ efforts have been taken on by new recruits vying to replace him as chief propagandist for ISIS.