Saudi Arabia is the “foremost” foreign funder of Islamist extremism in the U.K., according to new report released by a British think tank on Wednesday.

The Henry Jackson Society — a right-wing think tank — claims that overseas funding primarily from the governments and private charities of Gulf countries has a “clear and growing link” to the onslaught of violence the U.K. and other Western countries.

The group estimates that the Saudi government and charities spent an estimated $4 billion exporting Saudi Arabia’s strict interpretation of Islam, known as Wahhabism, worldwide in 2015, up from $2 billion in 2007. In 2015, there were 110 mosques in the U.K. practicing Salafism and Wahhabism compared to 68 in 2007. The money is primarily funneled through mosques and Islamic schools in Britain, according to the report.

“Influence has also been exerted through the training of British Muslim religious leaders in Saudi Arabia, as well as the use of Saudi textbooks in a number of the U.K.’s independent Islamic schools,” the report said.

Although many Western countries, including the United States, have acknowledged the threat of foreign terrorist financing, Britain “has seen far less of a response from policy makers supporting moves to tackle the challenge of foreign-funded Islamist extremism,” the report said.

Responding to the report, Labor MP Dan Jarvis told the BBC that “in the wake of the terrible and tragic terrorist attacks we have 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,” he said.

Prime Minister Theresa May created a private government commission for countering extremism in the wake of several deadly terrorist incidents in Britain, including the Manchester Arena attack in which a suicide bomber detonated during an Ariana Grande concert, killing 22 people. May has faced accusations that she is sitting on a report about terrorist financing that reportedly shows Saudi Arabia is largely to blame.

May has publicly reinforced the U.K.’s economic and security ties to the Kingdom and in turn, may be reluctant to point fingers. Saudi Arabia is one of the U.K.’s principal security partners in the Middle East, raking in $4.2 billion in weapons deals since March 2015.

Just like President Trump, one of May’s first international visits was to Saudi Arabia and she has repeatedly defended her relationship even in the face of criticism over the Saudi-led coalition airstrikes in the brutal war in Yemen.

The report also comes as Saudi Arabia and a number of other Gulf countries implemented a blockade against Qatar amid accusations that the country was itself a funder of terrorism. The report includes Qatar in its findings but still concludes that Saudi Arabia is the principal perpetrator.