The MP for Wallasey was a key shadow cabinet ally of Jeremy Corbyn until last month, when she launched a leadership challenge against him - though she has since bowed out in favour of another challenger, Owen Smith.
In a statement on his Facebook page, former Labour leader Ed Miliband urged party members to support Smith.
Yesterday, 44 female Labour MPs wrote to Mr Corbyn, calling for him to do more to combat abuse and threats made against women within the party.
Mr Smith accused Mr Corbyn of threatening MPs with the sack by raising the prospect of deselection, claiming that the Labour leader was bent on controlling the party.
It follows weeks of turmoil for the party nationally after the Parliamentary Labour Party adopted a vote of "no confidence" in the leadership of Jeremy Corbyn.
But in a thinly veiled threat to rebel MPs to toe the line or face the sack Corbyn announced a major shift in the party position.
"That is why I have reluctantly and sadly chosen to make this information public now", said McGinn.
"We've lost credibility, we've lost respect in many of our communities, communities that have been traditional Labour-voting areas".
Mr Corbyn will use the leadership contest to set out how a Labour government will tackle the "five ills" of 21st century Britain - inequality, neglect, insecurity, prejudice and discrimination.
This is huge. At least 180,000 people applied to join Labour in the 48-hour signing up window.
In his formal leadership speech, Corbyn took inspiration from social reformer William Beveridge and argued the Labour Party must confront "five evils" plaguing society.
"It's the job, it's the duty, it's the responsibility of every Labour MP to get behind the party", Corbyn said at the start of his leadership campaign.
Mr Smith said he had asked the leader to compromise "several times" in an attempt to stop the party from fracturing but had no success. They won them back-pay, but they also won them dignity and equality. "I hope they will recognise that and come on board".
Moderates fear the surge will back Mr Corbyn in the same way as 88,000 supporters paid £3 each to vote in last year's election.
"I don't do any abuse, I don't do any bullying, I don't allow it to be done anywhere to do with any of my campaign teams and I'm very surprised and very disappointed they should say that because politics has to be about bringing people in".