Ed Balls has blamed his stammer for his faltering response to the Autumn Statement, but George Osborne has accused the shadow chancellor of making excuses.
Balls appeared to fluff his lines in the Commons on Wednesday after apparently being caught out by news that borrowing had fallen this year rather than risen.
Speaking to BBC Radio 4's Today programme on Thursday morning, the shadow chancellor said his speech impediment sometimes "got the better of him" during parliamentary debates.
"What happens in the House of Commons when you are responding to that statement is you have none of the figures, none of the documentation, and you have to listen to the chancellor.
"The outside forecasters were all expecting a rise in borrowing this year, because it has risen for the first seven months ... it was impossible to work out in that first minute or two what was going on."
"Everybody knows with me that I have a stammer. Sometimes that stammer gets the better of me in the first minute or two when I speak, especially when I have got the Prime Minister, the Chancellor and 300 Conservative MPs yelling at me at the top of their voices.
He added: "But frankly that is who I am. I don't mind that."
But in a bad tempered interview later this morning on the Today programme, Osborne said he did not buy that excuse.
"The Commons doesn't take Ed Balls very seriously," he said."It's got nothing to do with the fact that he has got a stammer, it is because he was the chief economic adviser when it all went wrong, and he never acknowledges that.
"He never admits that he was there at the scene of the crime, and so obviously when we listen to his answers about what should happen next, we're a bit sceptical."
Balls has been trying to overcome his stammer since he was a child and spent years trying to conceal it. He is now the patron of the British Stammering Association.
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Conservative MP Claire Perry, who used to work as an advier to Osborne, said the Tory benchers "were laughing with incredulity at what Balls said, not how he said it".
But Labour MPs were not amused at the Tory laughter, claiming they were unfairly mocking Balls for his speech impediment.
Conservatives deny they were poking fun at Balls' stammer, rather at his inability to understand what Osborne had said.
It is not the first time MPs have been accused of making fun of someone for their disability. In February last year Labour MPs were said to have laughed at Paul Maynard, who has cerebral palsy, as he spoke.
In October 2010 Tory MPs were accused of of cheering the deepest cuts to public spending in living memory including to disability allowance.Suggest a correction