The crash, which happened at about 17:15 BST, has left almost 90 people hurt.
The BBC has been told that the transport secretary is among those advising Starmer to set out a timetable.
Discussion about the prime minister's future ramps up after Makerfield by-election.
Police say a man arrested on suspicion of attempted murder has been bailed.
After a slightly cooler Saturday, temperatures will rise again, possibly touching 35C by Tuesday with a Met Office amber extreme heat warning set to come into force.