GOP candidate says he will not release more tax records but his own review shows 'I never paid less than 13%' in the last decade

The Republican presidential candidate, Mitt Romney, has rejected a Democratic claim he had paid no tax over a 10-year period, saying that he had paid at least 13% a year.

But he again refused to release his tax returns as Democrats are demanding.

The Obama campaign said it was not satisfied with Romney's statement and repeated its calls for the tax returns to be released. "He has the ability to prove his claim," a spokesman said, adding: "We would say 'prove it governor Romney'."

Romney made his statement to reporters in response to a claim by the Democratic senate leader Harry Reid that he had been told by an informant inside Bain Capital, where Romney made his fortune, that he had paid no tax.

"I did go back and look at my taxes and over the past 10 years I never paid less than 13%. I think the most recent year is 13.6 or something like that. So I paid taxes every single year," Romney said.

He described the Reid report as totally false and challenged him to release the name of his informant.

During the Republican primaries and caucuses, under pressure from his party rivals, Romney released his full tax returns for 2010 and a summary of his tax return for 2011.

An Obama campaign spokesman said Romney's father, who was a governor and presidential candidate, had released 12 years' worth of tax returns and Romney should follow suit.

The White House spokesman, Jay Carney, said Romney should follow presidential precedent by releasing multiple tax returns.

Speaking at the daily press conference, Carney rejected suggestions that Obama is planning to remove Joe Biden as vice-president after a comment this week in which he told voters in Virginia if Romney became president, his approach to Wall Street would "put y'all back in chains".

The 2008 Republican presidential candidate John McCain, responding to a question about whether Obama should swap Biden for secretary of state Hillary Clinton, long touted as an alternative VP: "I think he might be wise to do that, but it's not going to happen obviously, for a whole variety of reasons – including in fact, if I'm Hillary Clinton I'm not sure I would want to be on that team."

Obama held a joint lunch on Thursday with Biden and Clinton. Carney said the Republicans knew what they were saying about Biden was ridiculous and they were only seeking to distract attention from policy issues.

Commenting on the chains remark, Carney said Biden had already "explained the use of his words, his language and how he had meant to phrase it. And I think I made the point that we all — all of us who are out there every day giving speeches, taking questions, talking about the issues, sometimes don't use the exact language that we thought we were going to use or wanted to use. But you know what he was talking about."