Anti-government protesters are planning "days of rage" in the run-up to Sunday's race, part of a 20-race circuit that generates annual revenues of $2 billion.

A former London policeman who is advising the Bahrain government said the safety of teams and spectators could not be guaranteed and an employee of the British-based Force India team returned home on Thursday because of security fears.

Andy Slaughter, a British politician who heads the All Party Parliamentary Group for Democracy in Bahrain, has written to several of the blue chip companies who bankroll the sport.

"The scheduling of the Bahrain Grand Prix will provide a forum and indicate to the rest of the world that it is business as usual - when the reality could not be further from the truth," he wrote.

"We are most alarmed that you see no grounds to sever your brand and save its reputation from a totalitarian regime," he added. "We sincerely hope you will rethink your associations with the Bahrain Grand Prix and decide to curtail your sponsorship of the race at Sakhir."

The letter was sent to Royal Dutch Shell, Vodafone, Unilever, Total, Siemens, Red Bull, UBS, News Corp, Hugo Boss, Ferrari, ExxonMobil, Deutsche Post and Daimler, Slaughter said.

"If they are major sponsors, they should at least defend their position," he told Reuters, adding that it was now probably too late to organise an effective boycott.

Thomson Reuters, parent company of Reuters news agency, sponsors the Williams Formula One team but Slaughter did not include it on his list of firms that were sent the letter.

A number of the Formula One teams are based in Britain and Briton Bernie Ecclestone, 81, runs the sport's commercial operations.

John Yates, a former assistant commissioner of London's Metropolitan police, said there were certain to be protests in Bahrain over the weekend.

"People say can we guarantee security. Of course we can't guarantee security. I'd be a fool to sit here and say that," he told Britain's Guardian newspaper.

(Reporting by Keith Weir; Editing by Peter Graff)

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