While Sir John said some re-elected governments’ popularity had tumbled fast, no newly elected – neither Labour nor Conservative – had crashed so far in the polls.

He said Sir Keir faces a similar position to John Major in 1992, whose popularity never recovered after Britain was forced to leave the Exchange Rate Mechanism on Black Wednesday. He lurched from crisis to crisis before losing the 1997 General Election by a landslide.

The latest YouGov opinion poll shows Labour on 24%, a fall of 11% from the 2024 election – nearly a third of their support from a year ago.

Nigel Farage’s Reform UK currently tops the poll with 26%, which YouGov has said would result in the insurgent party having the largest number of MPs in a hung parliament if an election were to take place now.

Speaking on TalkTV on Tuesday, Sir John added: “As compared with newly elected governments, this government saw its support fall more substantially and rapidly than any previously newly elected one.

“Of course, you do have to remember here a crucial point. This is a government that, despite that landslide majority, only got 35% of the vote. That is the lowest share of the vote ever won by any government with a majority.

“So it’s not as though this was a government that was elected with enthusiasm. A major reason for its majority is actually the movement of voters from Conservative to Reform, which then enabled Labour to pick up many Conservative-held seats.

“This was a government that started off with a relatively shallow basis of support and it’s just that we’ve discovered that it’s fallen away pretty rapidly.”

He also pointed out that Sir Keir “has never been popular with the public”, including as opposition leader.

The Prime Minister’s personal net approval ratings have tumbled since the election, hitting a low point of -46%. They have since recovered to -34%.