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.