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.