People in the UK are giving hundreds of thousands of pounds a year to Islamist extremists – sometimes unwittingly – in donations, a Government review has revealed.

Amber Rudd, the Home Secretary, said the “most common source of support for Islamist extremist organisations in the UK is from small, anonymous public donations, with the majority of these donations most likely coming from UK-based individuals”.

“In some cases these organisations receive hundreds of thousands of pounds a year,” she added.

“This is the main source of their income.”

Ms Rudd said donors may not know or support the organisations’ full agenda, but a short summary of the report released by the Home Office did not provide a breakdown.

“Some Islamic organisations of extremist concern portray themselves as charities to increase their credibility and to take advantage of Islam’s emphasis on charity,” it said.

“Some are purposefully vague about their activities and their charitable status.”

Home Secretary Amber Rudd claimed national security reasons and the personal information contained in the report meant it could not be released in full (PA)

The Home Secretary said she had decided not to publish the full “classified” report “because of the volume of personal information it contains and for national security reasons”, but said that Privy Councillors from opposition parties would be invited to read it.

The decision was attacked by political opponents including the shadow Home Secretary, Diane Abbott, who said the “public has a right to know” the governments, foreign and domestic groups and individuals who are funding extremism.

Green Party co-leader Caroline Lucas dismissed the summary as “utterly vague”, calling the decision “completely unacceptable”, while Liberal Democrat leader Tim Farron called on the Government to “name and shame” known culprits.

Haras Rafiq, CEO of the Quilliam counter-extremism group, accused the Government of “whitewashing” its findings, overlooking the alleged role of countries including Saudi Arabia, Qatar and Kuwait in constructing mosques where radicalisation takes place.

“This report may have identified charities and institutions that have received money from the British Government,” he told The Independent, naming the Gift Aid scheme as a potential contributor.

“National security is only impacted when you’re talking about terrorism – this report was not about terrorism, it was about Islamist and extremist funding.

“That excuse doesn’t really fly as far as I’m concerned.”

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Mr Rafiq said the Government could have redacted the names of individuals if needed but should “name and shame the groups that are radicalising our youngsters”.

“Why not name them? Who are they?” he added.

“If I as a British Muslim was to read that statement and say I don’t want to give money to those groups, how do I know who they are?”

Amid widespread concern over influence from Saudi Arabia and other Middle Eastern nations, the Home Office found that overseas funding was a “significant source of income” for a small number of suspected radical bodies, but “for the vast majority of extremist groups in the UK, overseas funding is not a significant source”.

It warned that foreign support has “allowed individuals to study at institutions that teach deeply conservative forms of Islam and provide highly socially conservative literature and preachers to the UK’s Islamic institutions”, adding: “Some of these individuals have since become of extremist concern.”

The summary said the Government would be “directly raising issues of concern, supported by evidence, with specific countries” but did not name them.

A spokesperson for the Home Office said no commitment had ever been made to publish the full results of the review, adding: “Contrary to suggestions by some media outlets, diplomatic relations played absolutely no part in the decision not to publish the full report.”

Theresa May with King Salman (left) during a visit to Saudi Arabia in April (Getty)

Islam instructs followers to give 2.5 per cent of their wealth (zakat) to the poor every year, with additional pushes for charity during the holy month of Ramadan.

As well as numerous Muslim charities in the UK, mosques are run by charitable foundations that raise money for both their own running costs and other causes.

Many groups advertise on Islamic television channels, which are regulated by Ofcom, while others use print and online advertising, or local community networks.

Two British charities were investigated last year after allegedly being used to raise money for al-Qaeda and Isis, under the guise of fundraising for victims of the Syrian civil war and helping Kurdish Muslims in Birmingham.

Several other organisations have come under suspicion over alleged ties with jihadis fighting in Syria, while some have been linked to Anjem Choudary’s banned al-Muhajiroun network and international Islamist organisation Hizb ut-Tahrir.

The Home Office report said regulation can improve transparency but some suspected extremist organisations are “seeking to avoid regulatory oversight”.

Its review, commissioned by David Cameron in November 2015, does not include the funding of terrorism or funding of extremism overseas from UK sources.

“It gives us the best picture we have ever had of how extremists operating in the UK sustain their activities,” Ms Rudd said.

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“Fundamentally, no single measure will tackle all the issues of concern raised in the review.

“A comprehensive approach focused particularly on domestic sources of support for all forms of extremism is needed.”

The Home Secretary outlined steps including public awareness campaigns to ensure donors understand the full intentions of organisations they choose to give their money to.

Information will also be given to the financial services sector, grant making trusts and foundations, so they can ensure they are not “inadvertently supporting” jihadis.

The Charity Commission will be among the bodies tasked with bringing a higher proportion of supposedly charitable groups under regulatory oversight, introducing a new requirement to declare overseas funding sources.

A spokesperson said the change was agreed following a consultation last year as part of wider work to combat “strategic risks facing charities” and increase accountability and transparency.

“Our plans to do this were already being proposed and developed before some of the issues raised by the government’s extremism funding review were highlighted,” she added.

“Charities are already required to declare their overseas expenditure.”