The Republican presidential candidate, taking a turn on the world stage, called London's problems with Olympic Games preparation "disconcerting." That prompted Cameron to retort on Thursday that doubters would "see beyond doubt that Britain can deliver." And London Mayor Boris Johnson told tens of thousands gathered in Hyde Park: "There's a guy called Mitt Romney who wants to know if we are ready. Are we ready? Yes we are!"
Amid the uproar, Romney tried to back off his critique, finally concluding, "I expect the games to be highly successful."
Romney also caused a stir with his attendance at a fundraiser with banking executives tainted by a British interest rate-fixing scandal. And he inadvertently disclosed that he held a secret meeting with the head of Britain's intelligence service.
The bobbles threatened to undermine Romney's first international tour as the man who would replace Democratic President Barack Obama.
A one-term Massachusetts governor with limited foreign policy experience, he is hoping to show voters back home that he is ready to represent the U.S. strongly and smoothly at a time of global economic turmoil and security troubles. He also wants to emphasize his own tenure running the 2002 Winter Games in Salt Lake City by attending this year's opening ceremonies. And he hopes to draw subtle contrasts with Obama.
"I'm looking forward to the bust of Winston Churchill being in the Oval Office again," Romney told donors at a fundraiser, Obama's administration sent back to Britain a bust of Churchill that once sat in the Oval Office. Obama aides say it was scheduled to be returned.
All in all, though, it was a shaky start to Romney's planned weeklong trip overseas that will include Israel and Poland after England.
As he met with British leaders past and present, Romney sought to keep the focus on the close alliance between America and Britain, praising "the unique relationship that exists between our nations, our commitment to common values, our commitment to peace in the world and a desire to see a stronger and growing economy."
But he may have ended up straining his own relationship with the British — and embarrassing them — by questioning whether the country could host a flawless Olympics after years of preparation.
England has constructed an enormous Olympic Park in East London, will showcase tennis at Wimbledon, is hosting soccer in Glasgow in Scotland and has even built a volleyball court behind No. 10 Downing Street, where Romney met with Cameron. Yet, the country has faced steady media coverage of things that have gone wrong: A security firm didn't hire enough people; problems at immigration threatened security risks.
Shortly after arriving in London on Wednesday, Romney told NBC News that it was unclear whether issues that have dogged the final preparations could be overcome, saying: "It's hard to know just how well it will turn out." He made the remark two days before the opening ceremonies and after his staff had attended the dress rehearsal for that show.
By the next morning, Cameron was standing outside Olympic Park responding to questions about Romney's comments.
"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 added. His aides said that was not intended as a jibe at Romney or Salt Lake City. But that didn't stop people on the Internet and elsewhere from suggesting it was.
As criticism of Romney's comments mounted, he suggested it's impossible for any Olympic Games to go off without a hitch.
Later, Romney and Cameron met in private, discussing economic issues and alliances in Afghanistan and Syria — as well as the Olympics — and the American sought again to clarify his remarks on the games.
"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."
By the end of the day Thursday, Romney was outlining his own mistakes as a way of explaining why he had mentioned problems with security forces and immigration enforcement.
"My experience as an Olympic organizer is that there are always a few very small things that end up not going quite right in the first day or so — these get ironed out and then when the games themselves begin and the athletes take over," Romney said as he stood outside No. 10 Downing St. "All the mistakes of the organizing committee, and I made a few, all of those are overwhelmed by the many things that the athletes carry out that capture the spirit of the Games."
Back home, White House spokesman Jay Carney piled on, telling reporters about an Olympic security briefing held in Washington.
"In keeping with our special relationship, the president also made it clear that he has the utmost confidence in our close friend and ally, the United Kingdom, as they finalize preparations to host the London Olympics," Carney said.
After a day of meetings and an interview with CNN, Romney headed to a fundraiser at the Mandarin Oriental hotel that raised over $2 million. It attracted employees of Barclays, which has been in the spotlight after becoming the first bank to admit its employees were involved in manipulating a key interest rate index.
Last month, U.S. and British agencies fined Barclays a total of $453 million. Chief executive Bob Diamond resigned. Diamond was to have been a host of Romney's fundraiser. He dropped out of the event but had already sent a check for $2,500.
So have 82 others who listed their employer as Barclays or Barclays Capital on Federal Election Commission records, including two who gave the maximum to the Romney campaign both in 2011 and 2012.
Reports of Barclays' links to the Romney campaign drew the attention of some members of the British House of Commons, who called on "Barclays and its executives to cease fundraising for political candidates immediately and to concentrate entirely on repairing confidence and trust in the banking system instead."
Romney took questions from the donors who had gathered in a ballroom at the hotel in the Knightsbridge neighborhood in London. Asked about how he would deal with banking regulation, Romney said the Republican Party has sometimes made a "mistake" in focusing too much on deregulation — but that a sweeping banking overhaul passed in the wake of the 2008 financial crisis went too far.
"I believe Dodd-Frank has gone beyond what was appropriate," Romney said of the banking bill.
Earlier, Romney met with the head of MI-6, Britain's intelligence service, during his visit to the Foreign Commonwealth Office. The meeting wasn't listed on public schedules, and revealing such a meeting isn't common in the U.K. But the Republican presidential candidate told reporters about it in a statement outside Cameron's home.
In his other meetings, British officials questioned Romney about so-called fiscal "cliff" that U.S. policymakers will have to deal with next year, as well as his plans to expand trade and potentially develop a free trade agreement between America and the Europeans. Romney asked questions about the situations in Iran and also in Syria, where a broiling conflict has the potential to flare as an election issue in the fall.
The candidate's troubles started even before he landed in London.
Earlier this week, the Australians issued a statement clarifying Foreign Minister Bob Carr's remarks after Romney told donors that Carr met with him and sees an "America in decline."
Romney also has faced criticism in the Jewish press abroad for scheduling a fundraiser on Tisha B'Av, a Jewish fasting day that commemorates the destruction of two temples in Jerusalem. And Romney was forced to distance himself from an unnamed campaign adviser quoted in the Daily Telegraph newspaper saying that Romney believes the U.S. relationship with Britain is special because of shared "Anglo Saxon heritage" and that the current White House doesn't appreciate that shared history.