WHEN David Cameron, Britain’s prime minister, announced on April 1st an investigation into the Muslim Brotherhood, Saudi Arabia was instantly suspected of putting the idea into his head. A few weeks earlier, the Saudis had formally declared the Arab world’s largest Islamist organisation to be a terrorist group. Across the Arab world, but especially in parts of the monarchical Gulf, it is under the cosh. In Egypt, where the Brotherhood was founded in 1928 by Hassan al-Banna, a schoolteacher, security forces have killed over a thousand members and supporters and have herded as many as 20,000 into prison. On March 24th an Egyptian court sentenced 529 people accused of being Brothers to death. The authorities say they carry out attacks, mainly against security forces, though jihadist groups far more militant than the Brothers have claimed most of them.

The new Saudi law will bolster Egyptian efforts to persuade other countries to follow the Saudis’ example in dubbing the Brothers as terrorists. But Gulf attitudes vary. In the 1950s many Brothers left Egypt for the Gulf after Gamal Abdel Nasser turned against them. They generally integrated into society, many as doctors, teachers and engineers.

In tandem with Saudi Arabia, the United Arab Emirates (UAE) have moved against them. Last year long sentences were handed down to Brotherhood-backed Islamists accused of plotting a coup. Official suspicion of Islamists has swept across the UAE.

The government of Kuwait, where the Islamic Constitutional Movement (ICM) takes part in the Gulf’s most open political system, deemed Egypt’s coup against the Brotherhood last year an “internal affair”. But local members of the group are nervous lest Kuwait’s rulers feel obliged to follow the Saudi suit. “The Kuwaiti authorities are smart, but there will certainly be pressure,” says Muhammad al-Dallal, an ICM member and former parliamentarian. A proposal to disband al-Islah, the Brothers’ Kuwaiti charity, is being contested in court.

Some in Kuwait say private donations to Syria’s Islamist rebels have dipped. Fearing that they may be accused of funding terrorists, some donors are waiting to see how other Gulf states will react. Bahrain, where the Brothers openly run charities and political groups, may be twitchiest, since its ruling royal family depends on Saudi help to suppress its Shia majority.

The Brothers’ Qatari branch was dissolved in 2003, yet the ruling house shelters prominent Brothers, such as Yousuf al-Qaradawi, a firebrand Egyptian preacher, and hosts Hamas, the Brotherhood offshoot that controls the Gaza Strip. That is the main reason the Saudis withdrew their ambassador to Qatar, prompting the UAE and Bahrain to do the same.