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US presidential candidate Mitt Romney has predicted a "very successful" Olympics, hours after talking of "disconcerting" signs in London.

He backtracked while meeting PM David Cameron, after telling US TV Britain may not be ready to stage the Games.

He said all Games saw "small" mistakes and he had seen "great progress" in the UK. But his remarks were later seized on by London Mayor Boris Johnson.

The Republican contender will take on Barack Obama in November's election.

In the contest for the Republican nomination, Mr Romney made much of the fact that he delivered a successful Winter Olympics in Salt Lake City in 2002, when he was chief executive of the event.

Speaking to reporters outside 10 Downing Street, he said he had been impressed by the enthusiasm of "millions of people across Great Britain" who had turned out for the torch relay.

Mr Romney said he "applauded the work of the organising committee in bringing the Olympic experience right into the heart of London".

Hours later, London Mayor Boris Johnson played on Mr Romney's earlier comments while revving up a crowd of thousands of people in London's Hyde Park.

Calling London "the greatest city on earth," Mr Johnson told the crowd: "I hear there's a guy called Mitt Romney who wants to know whether we're ready. Are we ready?"

'Not encouraging'

In an interview with NBC on Thursday, Mr Romney said stories about difficulties with security guards and threats of border staff strikes were "obviously... not something which is encouraging".

But following talks with Mr Cameron he said mistakes were to be expected and he was sure the Games would be a success.

"To look out of the back side of 10 Downing Street and see a venue having been constructed, knowing that athletes will be carrying out their activities almost in the back yard of the prime minister is really quite an accomplishment," said Mr Romney.

"My experience as an Olympic organiser is that there are always a few very small things that end up going not quite right in the first day or so.

"Those get ironed out and then when the games themselves begin and the athletes take over all the mistakes that organising committee - and I made a few - all of those are overwhelmed by the many things that the athletes carry out which capture the spirit of the Games.

"I don't know of any Olympics that has been able to be run without any mistakes whatsoever - but they are small and I was encouraged to see something that could have represented a real challenge, which was immigration and customs officers on duty, that is something which was resolved."

'Vote of confidence'

After watching the Olympic torch arrive in Downing Street, Mr Cameron described the US politician's comments as a "vote of confidence".

"He can see as everyone can see that this is going to be a fantastic Olympic Games in London."

Mr Romney, who will also visit Israel and Poland during a week long trip to boost his foreign policy credentials, earlier met former prime minister Tony Blair, Labour leader Ed Miliband and other senior figures.

The presidential candidate, who at one point addressed Mr Miliband as "Mr leader", said he recognised "the unique relationship that exists between our nations, our commitment to common values, our commitment to peace in the world and our desire to see a stronger and growing economy".

He spoke of the "common effort" towards peace and prosperity, adding: "I appreciate the work of the military of this great nation in our joint effort in Afghanistan, the people of Great Britain have sacrificed enormously in helping bring peace to that nation."

The former Massachusetts governor, who trails President Obama by an average of three points in opinion polls, also met former UK Prime Minister and Middle East peace envoy Tony Blair at Mr Blair's central London offices.

Mr Blair's spokesman said: "Ahead of the Governor's visit to the Middle East, they discussed the Middle East peace process, the situation in Syria, Iran and the wider region.

"The meeting then covered the economic challenges facing both Europe and the US. On the eve of the Olympic Games, the Governor and Quartet Representative also discussed their medal hopes for the US and British Olympic teams."