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.

Nonetheless, Mr Romney questioned the enthusiasm of the British public. "Do they come together and celebrate the Olympic moment?" he asked. "That's something which we only find out once the Games actually begin."

However, the presidential challenger later back-tracked on the comments at a press conference on Thursday morning.

"The weather could not be better. Fortunately the sunshine is out and the warmth is here," he said. "I know the spirit and the people of this community will welcome the athletes of the world."

Asked about the mix up between Korean flags at an Olympics womens' football match in Glasgow on Wednesday night, he said it was "impossible for mistakes not to occur", adding that "of course there will be errors from time to time".

Before meeting Mr Romney, Prime Minister David Cameron said that his main priority was to ensure a safe and secure Olympic Games and that police and security services were leaving nothing to chance just a day before the opening ceremony.

"This is the biggest security operation in our peacetime history, bar none and we are leaving nothing to chance," Mr Cameron told reporters at the Olympic Park.

The comments on the Olympics came after Mr Romney's advisers told The Daily Telegraph that he would restore "Anglo-Saxon" understanding to the special relationship between the US and Britain, and return Sir Winston Churchill's bust to the White House.

"We have a very special relationship between the United States and Great Britain," Mr Romney told NBC. "It goes back to our very beginnings - cultural and historical."

However, Mr Romney will receive a lukewarm reception in London. He met Tony Blair on Thursday morning at the former prime minister's London office and also held brief talks with Ed Miliband, the Labour leader.

In a gaffe of his own, Mr Romney referred to Mr Miliband as "Mr Leader".

“Like you Mr Leader I look forward to our conversations this morning and I have had a number of conversations with leaders both present and past and recognise of course the unique relationship that exists between our nations,” he said.

The current Government has a close relationship with President Barack Obama and the US administration took the unusual step of making this point during Mr Romney's visit.

A statement from Joe Biden, the Vice President, said: "Our special relationship with the British is stronger than ever and we are proud to work hand-in-hand with Prime Minister Cameron to confront every major national security challenge we face today."

Mr Romney, for his part, has portrayed Europe as a prime example of everything the United States must avoid. "We're at a point here where we have two different roads we can go down," he said on Wednesday. "One leads to Europe. The other leads to the kind of dynamism and prosperity which has always characterised America."

Later on Thursday, Mr Romney is planning to hold a fundraiser at the Mandarin Oriental hotel in Knightsbridge. One of the hosts of the fundraiser, former Barclays CEO Bob Diamond, withdrew from the event after he resigned in the wake of the Libor scandal.