The British hotels are the Dorchester and 45 Park Lane in London, and Coworth Park in Ascot, Berkshire.
Clooney wrote that a previous boycott of the chain’s Californian hotels had been “effective to a point …
The Dorchester Collection is owned by the Brunei Investment Agency, an arm of Brunei’s ministry of finance.
Saying he had no complaint against those who worked in the hotels, Clooney added: “I’ve learned over years of dealing with murderous regimes that you can’t shame them.