Eagle is challenging as she feels Corbyn would not be an effective challenger to the Conservative Party in a General Election.
Angela Eagle has formally announced her decision to stand for Labour leader - saying Jeremy Corbyn can not win a General Election.
Mr Mansfield's advice said: "The rules by which the Labour Party is governed are unambiguous: The leader does not require any signatures to be nominated in a leadership election where there is a potential challenger to the leadership".
Last week 172 Labour MPs, more than three-quarters of the parliamentary party, voted for a motion of no confidence in Mr. Corbyn's leadership.
Abbott said it would be "a travesty in terms of natural justice and fairness" if the party tried to keep Corbyn off the ballot. I'm a woman from the working class North - I understand metropolitan things too, ' she added.
He also dismissed suggestions from former shadow cabinet minister Owen Smith, who is thought to be considering his own leadership bid, that he would split the party.
If they rule that he is not eligible for automatic inclusion, then Mr Corbyn will have to garner the support of at least 20% of Labour MPs , which will prove hard after he lost a vote of no confidence among Labour MPs by 172 votes to 40.
Corbyn said he was disappointed with Eagle's bid to unseat him and would challenge any decision by the national committee to block him from standing again.
The National Executive Committee (NEC) voted in favour of Corbyn going straight onto the ballot, meaning he will not be required to secure nominations from 51 MPs and MEPs in order to fight for his leadership.
"These are dark times for Labour and these are unsafe times for our country", Eagle, the party's former business spokeswoman, said on Monday in a speech in London.
Describing herself as a "practical socialist", Ms Eagle said Mr Corbyn was not up to the job.
She said: "He has been hiding behind a door not talking to his Members of Parliament - that's not leadership". "I wouldn't do this if I didn't think I had something to offer to bring our party and our country back together".
"I am not contemplating losing this contest, I am in it because I want to win it for the best interests of a strong Labour Party", Eagle told ITV. "I will unite, I will not divide", Ms Eagle said.
Ms Eagle did not set out policy areas where she differs from Mr Corbyn, but said he can not win a general election.
Meanwhile, deputy leader Tom Watson told a meeting of the parliamentary Labour Party (PLP) that his abandoned peace talks with union leaders failed to find a way around the impasse between MPs and pro-Corbyn elements of the party.