Deputy Prime Minister John Prescott "blew his top" when questioned by a local reporter, a newspaper claims.
Mark Choueke questioning John Prescott. Pic: South Wales Argus
He told the journalist to "bugger off" and "Get on your bus, you amateur," the South Wales Argus reported.
Opposition parties criticised him, with a Tory candidate accusing him of "foul-mouthed, yobbish behaviour".
A spokeswoman for Mr Prescott said the journalist's approach was unreasonable and unprofessional. But the newspaper said that was "absolute nonsense."
The Argus said its reporter, Mark Choueke, clashed with Mr Prescott when the deputy PM was visiting a school in Cwmbran.
The journalist asked Mr Prescott about Peter Law - a Welsh assembly member who has just left the Labour Party to stand as an independent in the general election in his constituency of Blaenau Gwent.
The Argus said its reporter asked Mr Prescott as he walked back to his election bus from Croesyceiliog comprehensive how his cabinet colleagues reacted to Mr Law's decision.
It reported the exchange as follows:
John Prescott: It didn't even register with us. The voters just have one choice, vote Labour otherwise they'll end up with a Tory government. It's unfortunate that some of our decisions upset some people.
Mark Choueke: But this isn't about upsetting Peter Law, it's about upsetting the many thousands of Labour voters in Blaenau Gwent who helped you form a strong government - they feel alienated.
JP: Why are you asking me about this, I don't care, it's a Welsh situation, I'm a national politician.
MC: Are you too big to care about the Labour voters in Blaenau Gwent? Do you think there may be something in your party's methods of working that require a rethink when a politician chooses to stand against you after 35 years service to Labour?
The Argus denied its reporter was arrogant. Pic: South Wales Argus
JP: (walking away) Where do they get these amateurs from? You're an amateur mate, go get on your bus, go home.
MC: Are you too big for the regional press now John?
JP: Bugger off - get on your bus, you amateur.
MC: (following Mr Prescott) Is my interview over John? Because if that's all you've got to say, that's what will go in the paper.
JP: (turns back to reporter) - Ooohh, I'm scared, go ahead, put it in your paper.
Huw Edwards (Monmouth Labour candidate): "I could answer this question for you, Mark...
MC: I hoped to hear what the deputy prime minister had to say about it.
JP: (ignoring reporter) I've never seen a school in such a lovely setting.
Mr Prescott's spokeswoman said that he had already answered questions about Mr Law earlier.
She said: "In this particular situation the approach was quite unreasonable and quite unprofessional. There was just one question and the reporter kept going back and back on it."
Referring to Mr Prescott's "I don't care" remark, the spokeswoman said that had to be put in context.
She said: "Mr Prescott is not indifferent to this matter at all. He had answered questions quite repeatedly. The journalist was being quite arrogant and unreasonable about it."
Mark Templeton, news editor of the South Wales Argus, commented: "That is just absolute nonsense."
Monmouth Conservative candidate David Davies said: "The Labour Party must give an assurance that they must not send any more foul-mouthed politicians into the constituency for the rest of the campaign.
"We want a clean, fair fight in Monmouth and not the sort of yobbish behaviour the Conservative Party is trying to combat."
Mike German, leader of the Liberal Democrats in the assembly, said: "John Prescott clearly shows disdain for Labour voters.
"People of the south Wales valleys have been taken for granted for years and are now fed up with Labour. This is just another example of Labour treating them with contempt".
A Plaid Cymru spokesman described it as "typical New Labour arrogance".
"When it comes to Wales John Prescott always makes a big impact but unfortunately it's for all the wrong reasons. It is clear from his comments that Wales doesn't register on his radar."
An NOP poll conducted for ITV Wales suggests that Plaid Cymru could regain Ynys Mon from Labour.
The results of the poll: Eurig Wyn (Plaid Cymru) - 33%; Albert Owen (Labour) - 29%; James Roach (Conservative) - 21%; Sarah Green (Liberal Democrat) - 10%; Peter Rogers (Independent) - 6%; Elaine Gill (UKIP) - 2%; Tim Evans (Legalise Cannabis) - 1%.
Of those interviewed by NOP, 30% said they were undecided about who to support and 9% refused to state their voting intention.
The pollsters interviewed a representative quota sample of a thousand adults, by telephone, across Ynys Mon, between 14-18 April.