In an interview with NBC, the presumptive Republican presidential nominee suggested London might be "unprepared" for the games, which start Friday. A senior British government source said officials were stunned, The Guardian reported.
"What a total shocker," the source told the newspaper. "We are speechless."
London Mayor Boris Johnson, a Conservative, was far from speechless, calling out Romney by name as he spoke to a crowd estimated at 60,000 as the Olympic torch arrived in Hyde Park.
"I hear there's a guy called Mitt Romney who wants to know whether we're ready," Johnson said. "He wants to know whether we're ready. Are we ready? Are we ready? Yes, we are."
Prime Minister David Cameron responded initially with a joke about the 2002 Winter Olympics in Salt Lake City, which Romney managed: "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."
The joke prompted a rebuke from Salt Lake City Mayor Ralph Becker, whose office said in a statement to BuzzFeed: "While those of us who have had the fortune of visiting London know it is certainly a wonderful city, Prime Minister Cameron's comments likely reflect his lack of familiarity with Salt Lake City.
"He can stop by any time. We'd love to have him and are happy to send a map so he doesn't run into any trouble locating the middle of nowhere."
After spending 45 minutes with the prime minister at No. 10 Downing Street, Romney said he expected the games to be "highly successful."
Romney also met Thursday with former Prime Minister Tony Blair and Labor Party leader Ed Miliband in London. The former Massachusetts governor focused on the Olympics, rather than policy issues, in his meeting, The Washington Post reported.
"The athletes have arrived, the torch is about to come into London ... My experience is that this event will change the hearts of many, many people," Romney said.
Romney raised eyebrows among U.K. political analysts when he told reporters he had met with the head of the MI6 intelligence agency.
He finished the day with a fundraiser that brought in an estimated $2 million for his campaign, co-hosted by executives of banks being investigated as part of an interest rate-fixing scandal, the Post said.
"It has been a marvelous day for us," Romney said.
Two top Romney campaign surrogates -- Virginia Gov. Bobby Jindal and Virginia Bob McDonnell -- told reporters in a conference call they don't think the candidate's missteps in London will be a problem for his campaign.
"We're not worried about overseas headlines," Jindal said. "We're worried about voters back here at home in America."
Romney aides had said prior to his overseas trip U.S. Republicans were irked with Cameron's embrace of President Barack Obama.
Cameron, who visited the White House in March, joined the Democratic president on a visit to a college basketball game in Ohio -- a key swing state in November's presidential election -- and was later honored with a White House state dinner.
Cameron declined to meet any Republican leaders during the visit, the Romney advisers -- who requested anonymity -- told the British newspaper The Daily Telegraph.
Senior advisers to the presumptive Republican presidential nominee expressed similar criticisms to the British newspaper The Guardian in May, calling Cameron's White House visit a "love-in" with Obama.
One of the Romney advisers told the Telegraph Romney considered Britain "our oldest friend in the world," although Romney wrote in his memoir "No Apology": "England is just a small island. Its roads and houses are small. With few exceptions, it doesn't make things that people want to buy."
The Republican Party presented a united front with Britain's Conservative Party in the 1980s, when Ronald Reagan was president and Margaret Thatcher was prime minister. Since then, the political picture has become more complex, with the Labor Party in Britain moving to the right. Blair supported President George W. Bush's decision to invade Iraq when few other European leaders did.
Cameron's party is more in line with Democrats in several areas. He supports legalizing same-sex marriage and has called his administration the "greenest" in the nation's history.