Ian Austin said the Labour leader was "not fit to lead", and was joined by John Woodcock as they launched a campaign labelling Mr Corbyn a "disgrace to his party".
It came as the Jewish Chronicle newspaper published a stark front page urging voters not to support Labour because of Mr Corbyn's handling of anti-Semitism within the party.
The paper said the "near total inaction of Mr Corbyn and the rest of the Labour leadership in dealing with anti-Semites in the party has both emboldened them and encouraged others" - accusing Mr Corbyn and his allies of having "actively impeded action against the racists."
Mr Austin, who is not contesting the Dudley North seat he has held since 2005, issued a blistering attack on Mr Corbyn - telling the Express & Star newspaper: "I must do everything I can to stop Jeremy Corbyn from getting into power."
The long-time critic of Mr Corbyn and a former adviser to Gordon Brown quit Labour in February in response to what he claimed was a "culture of extremism, anti-Semitism and intolerance".
He said "decent patriotic Labour voters" should vote Tory to help Mr Johnson get the majority he needs to stop Mr Corbyn from entering Number 10, adding that the opposition leader was "too big a risk".
Shadow chancellor John McDonnell claimed Mr Austin was "employed by the Tories", adding: "What else do you expect him to do in an election campaign where you're employed by the Tories, you speak on behalf of the Tories. That's what this was about this morning."
Mr Woodcock, who announced this week that he would not stand for re-election in Barrow and Furness, "pleaded" with Labour supporters to "face up" to the choice of Mr Johnson or Mr Corbyn and to not "leave it until too late".
Speaking in Westminster, he warned: "Now all of that to an extent is water under the bridge, but we have arrived in a campaign where one of two people are going to be Prime Minister...
"The choice to keep Jeremy Corbyn away from Downing Street, to stop him getting his hands on the levers of national security and defence has to be to vote Conservative in this election and that's what I'll be doing as well."
Mr Woodcock quit Labour with a scathing attack on Jeremy Corbyn's leadership last year.
He had been sitting as an independent since having the Labour whip withdrawn in April 2018 pending investigation of an allegation, which he denies, of sexual harassment.
Their intervention came just hours after Labour's deputy leader Tom Watson - who has also been publicly critical of the leadership's attempts to tackle anti-Semitism in the party - announced he is standing down, saying the time was right for him to step away from politics.
After a turbulent start to the election campaign for the two main parties, Labour and the Tories sought to shift the focus to the economy instead.
Mr McDonnell said Labour would invest £150 billion in schools, hospitals, care homes and council housing over five years and he pledged an "irreversible shift in the balance of power and wealth in favour of working people".
"That means change, real change, and it means investment on a scale never seen before in this country and certainly never seen before in the north and outside of London and the south east," he told supporters in Liverpool.
"To achieve that objective it also requires, therefore, an irreversible shift in the centre of gravity in political decision making as well as investment in this country from its location solely in London to be relocated to the North and regions and nations of our country."
Chancellor Sajid Javid, speaking in Manchester, said the Conservatives would increase borrowing to pay for new infrastructure.
Heralding new fiscal rules for a "new economic era", he said: "While we must retain spending if we want growth to continue and get stronger in the future, then we need to invest in it. Taking the opportunity offered by those historically low borrowing rates."
In what could pose a significant threat to both main parties, the Liberal Democrats, Plaid Cymru and the Green Party announced they had agreed not to stand against each other in 60 constituencies across England and Wales.
The deal, brokered by the Unite to Remain group, will give voters a single Remain choice in 60 constituencies across England and Wales.
The group is confident that "at least 44" of the 60 seats are "highly winnable".
Elsewhere, the Prime Minister took to the skies on the first full day of his General Election tour which will see him visit Scotland, Northern Ireland and the north of England.
Mr Johnson visited a Tetley tea factory in Stockton and a whisky distillery in Scotland, where he said the Tories were the "party that is going to keep our fantastic United Kingdom together".
He denied suggestions of economic irresponsibility in the party's policies, claiming: "It's thanks to the prudence of the Conservatives that we are able to make some investments in our infrastructure, in technology, in our public services."
The PM added: "The biggest threat we face at the moment is the uncertainty that is still being caused to our political system by the whole Brexit debate."