Owen Smith called his Labour leadership rival Jeremy Corbyn “a lunatic” at a rally in west London.
Mr Smith was answering a question on how Labour could win back Tory voters and said: “At a hustings a few weeks ago, Jeremy Corbyn said, ‘Yes we’ve got to get some of the people who contemplated voting Tory in the past to vote Labour.’ Rubbish! We’ve got to get two million people who actually voted Tory, 12 months ago, to vote Labour, in 106 seats.
“And what you won’t get from me, is some, you know, lunatic at the top of the Labour Party, you’ll have someone who tries to form a coherent narrative about what’s wrong with Britain.”
Mr Smith also said he would fight a general election on a promise to keep Britain in the European Union if elected leader.
Almost all of the 106 seats Labour would have to win back from the Conservatives voted to leave the EU.
“Make party conference sovereign to make sure there won't be backsliding on the radical things I've promised,” Mr Smith said. “49 per cent of the people of this country didn't vote to leave the European Union. If we can win 49 per cent of the vote, we’ll be laughing.”
Mr Corbyn is firmly expected to win the competition, but Mr Smith said it was “50/50.”
“I’m telling you now, on the phones, this is 50 50 across the country. I was in Wolverhampton last night and we were winning hands down.
“There are 20 per cent of people who are undecided. I am telling you now at the end of this contest, I will have won it. “
One member said she would resign if Mr Corbyn won. Mr Smith told her: “If he wins, please don't resign, because we need to make sure we’ve got people fighting for our values to take on the Conservatives at the next election.”
Asked about his use of the word "lunatic" on BBC radio, Mr Smith later claimed he was in no way referring to Mr Corbyn.
He said: “I was saying that I wasn’t a lunatic. Having been accused earlier in the evening of running round like a lunatic, I was saying I wasn’t a lunatic.
“But if anybody is offended by the use of that word, then I do apologise. I’ve done that already this morning and I’ll do it again.
“But I wasn’t talking about Jeremy, I was talking about me.”
Mr Smith admitted that he was sometimes “colourful” with his language, but denied this was an impediment to him being leader.
A spokesperson for the Jeremy for Labour campaign, said:
"Owen Smith has degraded this contest by descending into personal abuse. He should apologise to people suffering with mental illness, many of whom would have been dismayed and upset to to hear such offensive language used in public by a Labour politician.
"He should also withdraw his remark, and spend time with people suffering from mental health problems to develop some sensitivity in his use of language. This is simply not the language that someone standing to lead our party should use, and it injects an ugly tone into this contest that no Labour member wants to see."
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