Emily Thornberry said a Labour government in a hung Parliament would seek support from other "progressive" parties – daring those who rejected the invitation to explain to their voters "why we have a Tory government instead".
The gaffe-prone Shadow Foreign Secretary's comments appeared to clash with Mr Corbyn's own more cautious approach at the same event when he was asked how far he would go with coalition-building to stop Theresa May returning to Downing Street.
The Prime Minister has repeatedly warned that voting Labour on June 8 risks putting a "coalition of chaos" in charge, with hardleft Mr Corbyn as Prime Minister propped by the Scottish National Party and Lib Dems.
Ms Thornberry was supposedly just chairing a question and answer session for Mr Corbyn after his speech to adoring supporters in Basingstoke.
Emily Thornberry said Labour will call on other parties to keep the Tories out of power
The truth is, we are fighting to win and we are fighting to win with a majority
But she leapt to the microphone to deliver her own opinion after her boss answered a question about the prospect of working with other parties after the election.
An excitable Ms Thornberry declared: "The truth is, we are fighting to win and we are fighting to win with a majority.
"If we end up in a position where we are in a minority, we will go ahead and we will put forward a Queen's Speech and a Budget.
She said a Labour government in a hung Parliament would seek support from ‘progressive’ parties
"And if people want to vote for it, then good.
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"And if they don't want to vote for it, they are going to have to go back and speak to their constituents and explain to them why it is that we have a Tory government instead.
"If we are the largest party, we go ahead – no deals – with our manifesto, our Budget and our Queen's Speech. That's it. No deals."
Mr Corbyn had answered a journalist's question, in the light of opinion polls suggesting Labour gaining ground, about whether he would go so far as to ask Sinn Fein MPs for the first time ever to take their seats if that was the way to kick Mrs May out.
Mr Corbyn said he never commented on polls: "The only poll I'm interested in is the one next Thursday."
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But there was "something very interesting" happening. The "cynics" who assumed at the start of the campaign that "nothing can change" had been confounded as "thousands and thousands and thousands" of people had gone to help fund and campaign for Labour.
"More than two million people have registered to vote who haven't voted before, many are young people. There is a mood out there about wanting something different," he went on.
"And so we're fighting every constituency to form a Labour government.
"We're not doing deals, we're not doing coalitions, we're not doing any agreements.
"We are fighting to win this election on a manifesto that I'm very proud of because it will fundamentally transform our society."
Theresa May has warned that voting Labour on June 8 risks putting a ‘coalition of chaos’ in charge
After Ms Thornberry's surprise intervention, during which the Labour leader looked worried, Mr Corbyn was pressed again on whether he ruled out coalition if the election produces an inconclusive result.
He replied: "Let's be clear. We're fighting every constituency in this election. That is the Labour Party's message. We've got a manifesto, we've got a programme. We're fighting this election to win.
"If you want a Labour government, vote Labour. That's the best way of getting it.
"Emily and I are of the same mind on this, that we are determined to win the election."
Conservative Party chairman Sir Patrick McLoughlin said: "The truth is out: Jeremy Corbyn will invite the other parties to prop him up as Prime Minister if there is a hung Parliament on June 9.
The latest opinion polls have suggested Labour is gaining ground
"Brexit negotiations start 11 days after you vote but instead of focusing on those negotiations Corbyn would be busy haggling with Nicola Sturgeon and the Lib Dems. That will cause chaos.
"Only Theresa May has the right plan for Brexit – and will use the opportunities it brings to build a United Kingdom that is stronger, fairer and even more prosperous.
"If you don't think Corbyn, propped up by the SNP and Lib Dems, is up to being Prime Minister, negotiating Brexit, or keeping our economy strong and nation secure, there’s no safe way to vote Labour wherever you live."
Labour is acutely sensitive about claims it might do deals with other parties after former leader Ed Miliband's 2015 general election bid was fatally undermined by Tory warnings which proved toxic on the doorstep against making him PM "propped up" by the SNP.
In previous interviews Mr Corbyn has refused to rule out doing a deal with Nicola Sturgeon's SNP.