The East of England Ambulance Service says nearly 90 people were injured, over 30 of them seriously.
Two East Midland Railway trains crashed into each other, injuring dozens of passengers and crew.
The BBC has been told that the transport secretary is among those advising Starmer to set out a timetable.
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.