Mr Romney, who was chief executive of the Winter Olympics in Salt Lake City in 2002, spoke on the first day of a visit to Britain.

He will meet the British political leadership in London on Thursday, including David Cameron, the prime minister, and Nick Clegg, the deputy prime minister. Mr Romney is then expected to attend the Olympics opening ceremony on Friday.

But he told US television there were “disconcerting” signs about Britain’s readiness. “It’s hard to know just how well it will turn out,” he said. “There are a few things that were disconcerting: the stories about the private security firm not having enough people, supposed strike of the immigration and customs officials, that obviously is not something which is encouraging.”

G4S, a private security company, provided thousands fewer staff than were expected, forcing the armed forces to fill the gap. British immigration officers had threatened to go on strike during the Olympics, but this ultimatum was called off on Wednesday. The Government insists that all preparations have been made and the event will be a success.