The decisions – particularly the “cut-off” point for people to join Labour in order to take part – could be crucial in settling who will lead the party’s fightback from its December general election disaster.
But Rebecca Long Bailey, the shadow business secretary and so-called “continuity Corbyn” candidate – and the early frontrunner – was absent from the weekend political shows and is still to confirm that she will run.
Her silence has prompted suggestions that her campaign is in trouble – and prompted Ian Lavery, the party chairman, to hint at a rival left-wing bid.
All except Mr Lewis appeared for setpiece interviews, criticising the overload of policies in Mr Corbyn’s defeated manifesto – while Ms Phillips said she would push to reverse Brexit if it proved too damaging.
The party’s current rules would give potential new members at least two more weeks to sign up with the right to vote, once the contest is officially launched this week.
It is thought that longer than a fortnight would favour ‘insurgent’ candidates, such as Mr Phillips or Ms Nandy – which in turn suggests the NEC, tightly controlled by Corbyn supporters, will reject the option.
In 2016, when the anti-Brexit Owen Smith, challenged Mr Corbyn, a bitter dispute over stopping new members from voting ended up in the courts.
The rules for “registered supporters” to take part will also be decided, allowing non-party members to sign up temporarily at a reduced cost to cast a vote.
In 2015, people were given two months to sign up for a small fee of £3 – but, a year later, the charge went up to £25 and they were given just two days.
The new leader is expected to be in place by the end of March, but there has been speculation that the contest could be shortened to, it is claimed, favour Ms Long Bailey as the establishment candidate.
As one of three shadow cabinet representatives on the NEC she is entitled to attend the London meeting, but is not expected to do so.
Meanwhile, Angela Rayner, a close ally of Ms Long Bailey’s, is expected to launch her bid for the deputy leadership on Monday – having disappointed supporters by opting not to pitch for the top job.