US Republican hopeful Donald Trump has lashed out at former nominee Mitt Romney after the 2012 candidate appeared to imply the billionaire had something to hide in his tax returns which was why they had not been published. Trump called Romney a "dope" as he launched his third attack on a senior Republican stablemate after previously attacking Jeb Bush and John McCain.

Romney made his comments ahead of "Super Tuesday" when the remaining Republican nominees take part in a five-way debate before 11 States go to the polls to decide who they believe would be the best candidate. Romney, beaten by the Democrats and Barack Obama in 2012, made the comments in an interview with Fox News, suggesting that either Trump wasn't paying enough tax or, for Trump a worse insult, that he's worth far less than the US$10bn (£7bn) he claims he's worth.

"We have good reason to believe that there's a bombshell in Donald Trump's taxes," said Romney. "Either he's not anywhere near as wealthy as he says he is, or he hasn't been paying taxes we would expect him to pay or perhaps he hasn't been giving money to vets or to the disabled like he's been telling us he's been doing."

Trump appeared to laugh off the comments, telling CNN "there is no bombshell at all other than I pay a lot of tax and the government wastes the money." However he also launched a savage attack on Romney, who had sought and got his endorsement in 2012 and who had himself had his tax affairs scrutinised after media pressure.

"Mitt Romney, who was one of the dumbest and worst candidates in the history of Republican politics, is now pushing me on tax returns. Dope!" tweeted Trump, adding: "I'm going to do what @MittRomney was totally unable to do - WIN!"

However Romney's attack comes amid unease in the Republican ranks that Trump is becoming unstoppable in his campaign to be endorsed. After a shaky start in New Hampshire he has won all three States since then. Next week 11 states will decide in the so-called "Super Tuesday" and if Trump performs well he may become the undisputed Republican candidate who would go up against Hillary Clinton or Bernie Sanders to be the next US President.