Mohamed Abrini, who became known as the "man in the hat" after he was spotted on CCTV footage following the Brussels airport attacks in March has become the first "ISIS supergrass" to assist British investigators, 'The Sunday Times' reported.
Following secret talks between Britain and Belgium, officers in Brussels asked Abrini questions provided by the UK authorities.
Abrini, who is facing terrorism charges in Belgium after being filmed at Brussels airport with two suicide bombers responsible for the attacks, has been helping British officers gather intelligence on suspected terrorist plots and jihadist sympathisers in the UK.
A "supergrass" is an informant or suspect who helps the authorities in return for immunity from prosecution, a more lenient sentence or a new identity.
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However, the evidence they provide may not always be reliable.
Abrini appeared in court in Belgium last week where a judge rejected his bid to avoid extradition to France, where he is suspected of involvement in the Paris attacks that killed 130.