A public inquiry must be launched into the Gulf states' funding of the Islamist extremism in Britain that is fuelling terrorism, according to a think tank.
A clear and growing link can be drawn between overseas money, which mainly comes from Saudi Arabia, and the recent wave of atrocities in the UK and Europe, the Henry Jackson Society said.
The kingdom's 60-year campaign to export hardline Wahhabi Islam has led to support for mosques and Islamic institutions that appear to have links to extremism, the organisation said.
It found there have been "numerous" cases of Britons who have joined Jihadist groups in Iraq and Syria whose radicalisation is thought to link back to foreign-funded institutions and preachers.
Prime Minister Theresa May, who visited Saudi Arabia earlier this year, has been accused of "kowtowing" to the kingdom by "suppressing" a report into the funding of extremist groups in the UK.
An inquiry was ordered in 2015 but reports have suggested the findings may never be published because of the sensitivity of the investigation's information regarding Saudi Arabia.
Labour MP Dan Jarvis said: "This report from the Henry Jackson Society sheds light on what are extremely worrying links between Saudi Arabia and the funding of extremism here in the UK.
"In the wake of the terrible and tragic terrorist attacks we've seen this year, it is vital that we use every tool at our disposal to protect our communities.
"This includes identifying the networks that promote and support extremism and shutting down the financial networks that fund it.
"I'm calling on the Government to release its foreign funding report, and guarantee that the new counter extremism commission will make tackling the funding of extremism a priority."
The Henry Jackson Society, which describes its approach to foreign and defence policy as "robust", called for a public inquiry into the issue.
Foreign funding for British extremism comes mainly from governments and state-backed foundations in the Gulf along with Iran, its study showed.
Report author Tom Wilson said: "There is a clear and growing link between foreign funding of Islamist extremism and the violent terrorism we have witnessed across the UK and Europe.
"The key now is to get ahead of the issue and find out the full extent of what has been going on. A public inquiry would go some way to informing the debate.
"While entities from across the Gulf and Iran have been guilty of advancing extremism, those in Saudi Arabia are undoubtedly at the top of the list.
"Research indicates that some Saudi individuals and foundations have been apparently heavily involved in exporting an illiberal, bigoted Wahhabi ideology."
A Government spokesman said: "Defeating the evil ideology of Islamist extremism is one of the greatest challenges of our time. The Commission for Counter-Extremism, which the PM announced earlier this year, will have a key role to play in this fight.
"We are determined to cut off the funding which fuels the evils of extremism and terrorism, and will work closely with international partners to tackle this shared global threat, including at the upcoming G20 summit."