Mitt Romney has emphasised his English ancestry after a gaffe-filled day in London, declaring himself to be “a guy from Great Britain” who is “married to a girl from Wales”.

Wearing a brave face after causing a diplomatic spat by questioning London's readiness to host the Olympics, the Republican presidential candidate joked that Britain felt like “home”.

The former Massachusetts governor and his wife, Ann, were interviewed together after a day from which he was meant to emerge looking like an international statesman descended into farce.

He told CNN's Piers Morgan on Thursday evening that the Olympics would be “terrific”, blessed with “great weather” and full of “enthusiasm on the part of the people here in London”.

The previous evening he told NBC that he had seen “disconcerting” signs that made it “hard to know just how well it will turn out” at the London games, prompting a sharp rejoinder from David Cameron.

Promising London would deliver, the Prime Minister said it had been challenging to host an Olympics in one of the world's busiest cities rather than the “middle of nowhere”, where it would be easier.

This was widely taken in the US to be a reference to Utah, the sparsely populated western state where Mr Romney was chief executive of the Salt Lake City Winter Olympics in 2002.

Boris Johnson, the Mayor of London, heaped more misery on Mr Romney's campaign by dismissing his remarks while onstage at a concert in Hyde Park to mark the end of the Olympic torch relay.

“There's guy called Mitt Romney who wants to know if we are ready,” Mr Johnson cried, prompting jeers from some of the 60,000-strong crowd. “Are we ready? Yes we are!”

By then a sheepish Mr Romney had deserted his earlier remarks with a statement delivered outside 10 Downing Street. He predicted that any minor problems would be “overshadowed by the extraordinary demonstrations of courage, character and determination by the athletes”.

Mr Romney was asked in his interview on Thursday evening whether he had been aware of his roots in northern England, where his great-great-grandfather, a carpenter from Preston, was one of the first Mormons in Britain some 175 years ago.

“I knew that my ancestors came from here,” he said. “I know Miles Romney and Miles Park Romney – these are the folks that came and helped settle the West.”

Asked whether he felt “partly English” as a result, a chuckling Mr Romney replied: “Well, I’m married to a girl from Wales, and I’m a guy from Great Britain. So I feel like this is home too, I guess.”

His wife is descended from Welsh coal-miners. Her grandfather, David Davies, travelled to Michigan from Wales in 1929.

Mrs Romney told Morgan that the ITV drama Downton Abbey was one of their favourite television programmes, and that the couple were currently working their way through the second series.

The Republican challenger also raised eyebrows on Thursday by referring to Ed Miliband with the American-style honorific “Mr Leader” and appearing to breach protocol by disclosing that he had met Sir John Sawers, the Secret Intelligence Service chief.