Jeremy Corbyn and Angela Eagle are due to appear on TV to set out their credentials for the Labour leadership.
Ms Eagle plans to announce her bid for the leadership on Monday, triggering a vote by the wider party membership.
Labour’s National Executive will decide later whether Mr Corbyn must seek fellow MPs’ nominations for his name to appear on the ballot paper.
Union leader Len McCluskey has warned of “lasting division” in the party if Mr Corbyn’s name does not appear.
BBC correspondent Iain Watson says Mr Corbyn’s supporters say they have legal advice that he should be on any ballot automatically as the incumbent – but some party officials believe he would have to seek nominations from fellow MPs, as will Ms Eagle, the former shadow business secretary.
If the National Executive accepted this, and Mr Corbyn did not receive enough support in Westminster, he may be excluded from the leadership race before party members can cast a vote on his future, our correspondent added.
A vote of no confidence in Mr Corbyn by Labour MPs last month was passed by 172 votes to 40. But in last year’s leadership election Mr Corbyn was elected by the wider membership on the first ballot with almost 60% of the vote.
By Iain Watson, BBC political correspondent
All week, Angela Eagle has tried to persuade Jeremy Corbyn to stand down voluntarily, but he has said he won’t betray the party members who elected him.
So she’ll trigger a leadership contest on Monday, as she has the necessary support of 50 of her fellow MPs.
Labour’s ruling National Executive is likely to meet 24 hours later, and they will have to take a decision which could have serious consequences for their party’s future.
Labour party officials say they have legal advice which suggests he would have to seek nominations from fellow MPs, just like his challenger.
If the National Executive accepts this, and Mr Corbyn then fails to get enough backing at Westminster, he may be out of the race before party members get a say.
The leader of the Unite union, Len McCluskey, has warned that this would risk “a lasting division in the Labour party”.
Privately, some of Mr Corbyn’s opponents who previously saw a split as undesirable, now regard it as almost inevitable.
Mr McCluskey, head of the Unite union which is Labour’s biggest donor, said: “I must warn that any attempts to keep Jeremy Corbyn, elected just 10 months ago with an enormous mandate, off the ballot paper by legal means risks a lasting division in the party.
“It is time for everyone to commit to a democratic and dignified procedure as the only way to avert such a disaster for working people.”
The developments come after deputy party leader Tom Watson called off talks aimed at ending the crisis in Labour.
Unite had been due to meet representatives of Mr Corbyn and Labour MPs in Brighton on Sunday,
Mr Watson said there had been “significant progress” during initial talks in recent days, adding it was his “strong belief” the discussions could have led to a basis for further talks.
But he said: “It is with regret and profound sadness that I have concluded there is little to be achieved by pursuing wider conversations with our union affiliates at this time.”
Mr Corbyn’s declaration to continue “come what may” meant there was “no realistic prospect of reaching a compromise” over the leadership, he said.
Mr McCluskey accused Mr Watson of an “act of sabotage fraught with peril for the future of the party”.