Mitt Romney leaves 10 Downing Street after meeting David Cameron. Photograph: Oli Scarff/Getty Images
Mitt Romney handed Barack Obama a priceless gift for the US presidential election campaign when the presumptive Republican nominee blundered on his first diplomatic outing by questioning whether London was capable of staging a successful Olympic Games.
In a move that astonished Downing Street, hours before it laid on a special reception for Romney at No 10 he told NBC there were "disconcerting" signs about the preparations for the Games.
One senior Whitehall source said: "What a total shocker. We are speechless."
David Cameron wasted no time in slapping down Romney hours after his remarks were broadcast. On a visit to the Olympic Park, the prime minister said: "We are holding an Olympic Games in one of the busiest, most active, bustling cities anywhere in the world. Of course it's easier if you hold an Olympic Games in the middle of nowhere."
Cameron's remarks were intended to be a light-hearted rebuke to Romney, who used his famous management skills honed at Bain Capital to rescue the Salt Lake City Winter Olympics in 2002.
Romney frantically rowed back later after a 45-minute meeting in Downing Street where the prime minister expressed his unease about his remarks. "I am very delighted with the prospects of a highly successful Olympic Games. What I have seen shows imagination and forethought and a lot of organisation and [I] expect the Games to be highly successful," he said.
When asked about the preparations for the Olympics earlier in the day by the NBC anchor Brian Williams, Romney said: "You know, it's hard to know just how well it will turn out. 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. Because there are three parts that makes Games successful.
"Number one, of course, are the athletes. That's what overwhelmingly the Games are about. Number two are the volunteers. And they'll have great volunteers here. But number three are the people of the country. Do they come together and celebrate the Olympic moment? And that's something which we only find out once the Games actually begin."
Boris Johnson also waded in to reject Romney's claim. "London is as ready as any city has ever been in the history of the Olympic Games," the London mayor said.
Romney's remarks are likely to be used by the Obama campaign to highlight his lack of sensitivity and his inexperience on the world stage. There was irritation in No 10 because Cameron had gone out of his way to make Romney as welcome as possible without breaching strict protocol rules.
Romney was given an important photo opportunity by being allowed to walk through the famous No 10 front door, though he had to do this on his own because he is not a head of government or state. Once inside the prime minister laid out the diplomatic red carpet. There was a warm handshake for the cameras before they held talks for 45 minutes about the Olympics, Syria, Iran, Egypt, Tunisia, Libya, Pakistan and Afghanistan.
Earlier, Romney appeared to forget Ed Miliband's name when they met at Westminster. "Like you, Mr Leader, I look forward to our conversations this morning," Romney said to Miliband as they shook hands.
Labour sources were relaxed about the "Mr Leader" reference, which revived memories of Neil Kinnock's notorious visit to the White House when Ronald Reagan referred to Denis Healey as Mr Ambassador. A Miliband aide said US politicians always referred to fellow politicians by their job title.
On his first official diplomatic trip since he became the presumptive Republican candidate, Romney held a series of "grip and grin" meetings with political leaders. He started the day meeting Tony Blair – the most recognisable political figure in the US – to brief him before Romney's visit to Israel.
He also met Nick Clegg, William Hague and George Osborne. One meeting was held way from the cameras when Romney was briefed by Sir John Sawers, the chief of the Secret Intelligence Service, MI6. This prompted Romney's third blunder of the day when Romney announced in Downing Street that he had met Sawers. Visiting dignitaries tend not to announce when they meet the head of MI6.