Andy Burnham has been warned not to get too carried away with Labour's victory in the Holborn & St Pancras by-election.
The party's candidate Sagal Abdi-Wali romped home with a 3,000 majority in the contest for the seat vacated by Keir Starmer.
The result has left Green leader Zack Polanski struggling to cling on, amid signs that his popularity has already faded.
However, elections guru Sir John Curtice said the PM would be wrong to draw the conclusion that Labour is dominant again.
Prof Curtice told the BBC that the outcome in Holborn reflected that the Greens were 'notably weaker' in national polls than the spring, when they made big gains in local elections.
But they remain 'more significant players than they have ever been'.
Andy Burnham has been warned not to get too carried away with Labour's victory in the Holborn & St Pancras by-election
Ms Abdi-Wali's 45 per cent of the vote was down four points on Sir Keir's support in 2024, which in itself was much lower than he secured in 2019 and 2017.
Prof Curtice said Holborn underlined that Labour was in a better position than before Mr Burnham became leader, but still less popular than two years ago.
In that context calling a snap election would be 'an extremely risky project'.
'This is not a result that is indicative of Labour being able to do well enough to be able to win a general election with an overall majority,' he said.
Mr Burnham has been furiously trying to quell rumours of a snap ballot, declaring the idea 'dead' before Labour conference last week.
But he used his big speech in Liverpool last week to unveil policies on social care, electoral reform, the state pension 'triple lock' and Brexit, which would all only happen after an election.
Kemi Badenoch used the Tory gathering this week to urge her party to be ready for the nation to go to the polls in May.
Ms Abdi-Wali, who is also the leader of Camden Council, was the bookies' favourite
An Opinium survey released at the weekend showed Labour support rising to 30 per cent, the highest since 2024.
The latest YouGov research has found Labour's support was up three points from before its conference, reaching 27 per cent.
That was five points clear of Reform, which was down by the same proportion, while the Tories were in third on 20 per cent.