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Last October Mr Corbyn, who is favourite to win the Labour leadership when the results are announced on Saturday, was one of the signatories of a ­letter defending the activities of the charity Cage.

It hit the headlines in March after its leader Asim Qureshi described Mohammed Emwazi, known as Jihadi John, as a “beautiful young man”.

The charity has a long history of supporting extremists, including radical Muslim cleric Abu Hamza and Aafia Siddiqui, dubbed “Lady Al Qaeda”.

The letter was published in a national newspaper just days after the charity’s director Moazzam Begg, a former Guantanamo detainee who was released without charge in 2005, walked free from jail after seven terrorism-related charges against him were dropped. Mr Begg, 46, has now endorsed Mr Corbyn in the leadership race.

The letter attacked the Government for freezing the charity’s assets and bank accounts and urged the Charities Commission to drop the investigation it had launched into some of its donors. Last night ­Muslim MP Khalid Mahmood criticised Mr Corbyn, who has already come under fire for his ­connections to anti-Semites.

Mr Mahmood, Labour MP for Birmingham Perry Barr, said: “Cage is an organisation that has a very chequered past and anybody who sympathises with them fails to understand the seriousness of the challenges we face at this moment in terms of countering radicalisation.

“I don’t think it would be appropriate for an MP, let alone the leader of the Labour Party, to be involved with this organisation. I think Jeremy Corbyn needs to think more carefully about the kind of people and organisations to which he gives his support.

“We already face a huge ­challenge ­trying to stop terrorists and ­people who are trying to ­radicalise others and yet this organisation has endorsed these ­people.” Robin Simcox, research fellow at the Henry Jackson Society, an anti-extremism think tank, said: “Cage is a hugely problematic organisation and has been for some years. They are involved with and have backed various convicted terrorists.

“They are involved with people like Al Qaeda cleric Anwar Al Awlaki and have a host of other undesirable associations.

“It is an organisation that any politician should think carefully about before allying themselves with it.”

Aafia Siddiqui was jailed for 86 years in America for attempting to murder US officials in Afghan­istan.

She has wide-ranging links with Al Qaeda and was married to a key plotter behind the 9/11 attacks in New York and Washington.

Yet Cage’s profile on Siddiqui omits all this and says it has “dedicated itself” to freeing her.

The Sunday Express made repeated attempts to contact Mr Corbyn’s team but no one was available for comment yesterday.