Saudi Arabia has been described as the “foremost” financier of Islamist extremism in the U.K. by a right-wing think tank.

The report, published by the Henry Jackson Society, citing multiple media outlets, right-wing U.S. think tanks and other websites, links several Saudi charities and organizations to a growth in the number of British citizens becoming radicalized and leaving the country to fight for the jihadist militant group Islamic State (IS) in Iraq and Syria. It said that the form of education advanced by such organizations promotes a “hardline Wahhabi interpretation of Islam endorsed by the Saudi state”.

It has called for a public inquiry into the funding of extremism in the U.K. by Gulf states. It also claims Iran to be one of the Arab states involved in the financing of extremist groups.

“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,” Tom Wilson, author of the report said on Wednesday. “Research indicates that some Saudi individuals and foundations have been apparently heavily involved in exporting an illiberal, bigoted Wahhabi ideology.”

Two of the organizations it singled out as promoting “violent Islamist extremism” were the World Assembly of Muslim Youth (WAMY) and the Muslim World League (MWL) which, it said, have links to the Muslim Brotherhood.

Saudi Arabia is the closest ally to the U.K. in the Middle East.

At the time of publication WAMY and MWL had not responded to requests for comments.

An opposition lawmaker said the report was “extremely worrying” and called on the U.K. Prime Minister Theresa May to publish an inquiry into the funding of extremist groups in the U.K., which was ordered by her predecessor David Cameron in 2015.

“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,” Dan Jarvis, member of parliament for the U.K.’s centre-left Labour Party said. “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,” he added.

Labour leader Jeremy Corbyn criticized the U.K. Prime Minister Theresa May last month for “suppressing” the report. “We have to get serious about cutting off the funding to these terror networks, including Isis here and in the Middle East,” he said in a speech focused on the London Bridge terror attack.

“Defeating the evil ideology of Islamist extremism is one of the greatest challenges of our time,” a Government spokesman told CNBC over the phone.

He added: “We will work closely with international partners to tackle this shared global threat, including at the upcoming G20 summit.”

The U.K. Saudi Arabia embassy told CNBC: “The claim that the Kingdom of Saudi Arabia finances Islamic extremism in the UK is categorically false. The Kingdom of Saudi Arabia affirms, once again, that it has not and does not support or fund any group that has direct or indirect links to any terrorist organization.”

“Terrorist ideology knows no nationality, language or borders,” it added. “All charitable donations to educational and religious establishments by the Saudi state are made to registered charities in the U.K. Saudi charities are prohibited from transferring money abroad and cannot operate abroad except through the King Salman Humanitarian Aid and Relief Centre or the Saudi Red Crescent, which is a semi-government entity.

“Saudi Arabia has been at the forefront of fighting the spread of extremism and terrorism at home and abroad. Saudi actions at home are indicative of the approach it encourages for other nations abroad. We continue to work alongside our allies against radical extremists.”

The embassy added that IS and other extreme religious groups had been blacklisted as terrorist organizations, and that participation in and funding of those groups was criminalized “at home and abroad”.