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.
The Henry Jackson Society 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.
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."
According to Wilson, entities in Saudi Arabia are at the top of the list of countries advancing extremism, being involved in "exporting an illiberal, bigoted Wahhabi ideology."
The study also revealed that 24% of Islamism-related offences between 1998 and 2015 had military targets, including British or coalition forces.
Foreign funding for British extremism comes mainly from governments and state-backed foundations in the Gulf along with Iran, its study showed.
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."