Gordon Brown is set to learn whether he will face a contest to succeed Tony Blair as Prime Minister.
Left-wing leadership hopefuls John McDonnell and Michael Meacher will meet to see if they can muster enough nominations between them for one of them to secure a place on the ballot paper.
The two have agreed that the one with the most nominations by Labour MPs should go forward as the candidate of the left.
However, at an initial meeting on Friday they failed to agree who that should be, prompting speculation that they were struggling to get the 44 names needed.
Whatever the outcome of their negotiations, there is little doubt at Westminster that Mr Brown is certain to win the race to No. 10.
The battle to be deputy PM is expected to be far closer. Three of the six hopefuls - Northern Ireland Secretary Peter Hain, party chairman Hazel Blears and justice minister Harriet Harman - have already said they have the 44 nominations needed to go forward as candidates.
Education Secretary Alan Johnson is also expected to announce this week that he has the votes in the bag to get his name on the ballot paper.
International Development Secretary Hilary Benn and backbencher Jon Cruddas are also said to be confident - although whether they will both qualify remains to be seen.
All the candidates have until Thursday, when nominations finally close.
