US Republican presidential candidate Donald Trump in Ames, Iowa, at the weekend, where he criticised John McCain

Donald Trump is unfit to be president, his rivals have warned, after the billionaire caused new controversy with a dismissal of Senator John McCain’s record as a war hero.

Mr Trump refused last night to back down after he ridiculed Senator McCain, who was imprisoned in Vietnam, saying: “I like people who weren’t captured.”

Republican presidential contenders Florida Senator Marco Rubio and former Texas Governor Rick Perry immediately branded Trump unsuitable to run for the presidency.

“I will say what I want to say,” said Trump, claiming a strong record of supporting veterans and accusing Senator McCain of failing them in Washington.

“I will do far more for veterans than John McCain has done for many, many years, with all talk, no action,” Trump said. “He’s on television all the time, talking, talking. Nothing gets done.”

Senator McCain refused to comment on the accusations.

Although unrepentant, Mr Trump conceded that Senator McCain might be a hero after all, but said people who “fought hard and weren’t captured and went through a lot, they get no credit.” He insisted he would continue to pursue the Republican nomination: “I’m certainly not pulling out.”

Senator McCain spent more than five years as a prisoner of war, enduring torture. He stirred Mr Trump’s anger last week when he said Mr Trump’s comments about immigrants had “fired up the crazies”.

Weeks ago, after Mr Trump asserted that Mexican immigrants are rapists and drug dealers, Hispanic leaders were incensed not only about those remarks but about the slow and halting response from others seeking the Republican nomination. But the fallout from Mr Trump’s latest salvo has spread quickly and indicates that at least some of his competitors are losing their inhibitions about distancing themselves from him.

Senator Rubio said yesterday that Mr Trump had insulted all prisoners of war, not just McCain, the 2008 Republican presidential nominee defeated by Barack Obama.

“He’s saying that somehow if you’re captured in battle, you’re less worthy of honours,” Senator Rubio said. “It’s not just absurd, it’s offensive. It’s ridiculous. And I do think it’s a disqualifier as commander in chief.”

Senator Rubio said that as the campaign goes on and Mr Trump commands attention, “it’s required people to be more forceful in some of these offensive things that he is saying.”

Mr Perry, one of the few military veterans running for president, said Mr Trump had demonstrated he had neither the character nor the temperament for the White House. “Over the top,” the former Texas governor said. “Really offensive.” Meanwhile, Jeb Bush, whose wife is from Mexico, took sharp offence at Mr Trump’s earlier comments as others hedged their bets.

After Mr Trump’s latest comments, McCain, the former Florida governor, tweeted, “Enough with the slanderous attacks.”

But both Texas Senator Ted Cruz and former Arkansas Governor Mike Huckabee, while agreeing Senator McCain is a genuine hero, sidestepped when asked if they condemned Mr Trump’s remarks.