Islamist extremist organisations are receiving hundreds of thousands of pounds a year, mainly made up of small, anonymous public donations from UK-based individuals, a long-awaited Government report has found.
Home Secretary Amber Rudd said she had decided against publishing the report in full for reasons of national security and because of the volume of personal information it contains.
She 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."
She said the donors may not know or support the organisations' full agenda.
The report was commissioned by then prime minister David Cameron in November 2015.
The Home Office review into the nature, scale and origin of the funding of Islamist extremist activity in the UK also looked at overseas sources.
It found that for a small number of organisations with which there are extremism concerns, overseas funding is a "significant source of income".
But it found that for the vast majority of extremist groups in the UK, overseas funding is not so important.
Ms Rudd said the review "gives us the best picture we have ever had of how extremists operating in the UK sustain their activities".
Other findings include that some Islamic organisations of concern are posing as charities to increase their credibility and to take advantage of Islam's emphasis on charity.
Some of these are deliberately vague about their activities and their charitable status.
Overseas support has allowed individuals to study at institutions that teach "deeply conservative forms of Islam", the report found.
Overseas support also provides "highly socially conservative literature and preachers" to the UK's Islamic institutions and some of these individuals have since become of extremist concern.
Ms Rudd said the Government would be directly raising issues of concern with "specific countries as part of our wider international engagement on countering extremism and violent extremism".
She said no single measure will tackle all the issues raised in the review and a comprehensive approach focused particularly on domestic sources of support for all forms of extremism is needed.
She said, after analysing the findings, the Government will also build on existing work by continuing to deliver public awareness campaigns to encourage people to understand the full aims of the organisations that they give to, and raising awareness across the financial services sector to ensure they are not inadvertently supporting extremist individuals or organisations.
The Government will also work with the Charity Commission to address the abuse of charities for terrorist or extremist purposes.
The Charity Commission will be introducing a requirement on charities to declare overseas funding sources, Ms Rudd added.
The report was criticised by Green Party co-leader Caroline Lucas, who said it gave no details of which countries funding for extremism comes from.
She said: "The Government's refusal to publish this report, and this utterly vague statement, are completely unacceptable.
"The statement gives absolutely no clue as to which countries foreign funding for extremism originates from - leaving the Government open to further allegations of refusing to expose the role of Saudi Arabian money in terrorism in the UK.
"The Government accept that foreign funding is a significant source of income for some extremist groups here in Britain - but they won't say in public where that money is coming from.
"It's not good enough to simply let privy councillors see this report - because such a rule excludes party leaders like myself who are trying to hold this Government to account on this issue and shine a spotlight on the deep complicity between Whitehall and Riyadh."
Liberal Democrat leader Tim Farron also said further detail was needed.
"The bottom line is that this report still remains unpublished," he said.
"The Government hoped to sneak out what it thinks are the top lines of a report no-one has seen on a busy day, hoping no-one would notice. This decision is utterly shameful.
"I believe that sunlight is the best disinfectant and the Government should release this report today and stop it gathering dust in a draw in Number 10.
"We cannot tackle the root causes of terrorism in the UK without full disclosure of the states and institutions that fund extremism in our country.
"Instead of supporting the perpetrators of these vile ideologies, the Government should be naming and shaming them - including so-called allies like Saudi Arabia and Qatar if need be.
"It seems like the Government, yet again, is putting our so-called friendship with Saudi Arabia above our values. This shoddy decision is the latest in a long line where we have put profit over principle."