Mitt Romney, in his first trip abroad as the presumptive GOP presidential nominee, is trying to walk back comments he made questioning London's Olympics preparation -- comments that have drawn a sharp response from Prime Minister David Cameron.

The dustup began Wednesday, as Romney, who ran the 2002 Salt Lake City games, said there were "disconcerting" signs in the days before this year's games.

"The stories about the private security firm not having enough people, the supposed strike of the immigration and customs officials -- that obviously is not something which is encouraging," he told NBC News.

"Do they come together and celebrate the Olympic moment? And that's something which we only find out once the games actually begin," he said.

Cameron soon rebuked Romney . "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," he said.

"I think we will show the whole world not just that we come together as a United Kingdom, but also we're extremely good at welcoming people from across the world," Cameron added. "I will obviously make those points to Mitt Romney. I look forward to meeting him."

In comments before meeting with Labour Party leader Ed Miliband, Romney was more measured. "My experience with regards to the Olympics is it is impossible for absolutely no mistakes to occur," he said. "Of course, there will be errors from time to time, but those are all overshadowed by the extraordinary demonstrations of courage, character and determination by the athletes."

UPDATE: 3:35 p.m. -- Mitt Romney's disastrous British trip continued Thursday when, according to The Huffington Post UK , he "caused amusement" by saying he had spent a great day in the "backside" of Downing Street, rather than the back garden.

In another faux pas, Romney announced his meeting with MI6, the U.K. Secret Intelligence Service whose existence was only acknowledged by the British government in 1994.

"I appreciated the insights and perspectives of the leaders of the government here and opposition here as well as the head of MI6," he told reporters.

MI6 was mum about the meeting, according to the Wall Street Journal . "[MI6 Chief] Sir John Sawers meets with lots of people," said an aide in the British foreign press office, "but we dont give a running commentary of any of these meetings."

London Mayor Boris Johnson hit back at Romney's comments about the Summer Games before a crowd of 60,000 in Hyde Park. "There is a guy called Mitt Romney who wants to know if we are ready. Yes, we are," he declared .

Romney also seemed to break the longstanding rule for U.S. politicians not to criticize the president overseas. At a fundraiser for American expats, he reportedly said, "I'm looking forward to the bust of Winston Churchill being in the Oval Office again," referring to the White House returning the artwork to the British Embassy in early 2009. President Barack Obama replaced it with a bust of Abraham Lincoln.