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The upcoming contest could allow the Prime Minister to judge whether he has seen off the threat from the left

The by-election in Sir Keir Starmer's old seat has been described as Andy Burnham's "test run" for an early general election.

If Labour snatches a resounding win in the Holborn and St Pancras mid-term, where the party is facing off against Greens leader Zack Polanski, Mr Burnham could bring foward the 2029 election.

The contest was triggered when Sir Keir announced his resignation at the start of September - despite previously vowing to stay on as an MP following his departure from No10.

Now, Mr Burnham's advisers have suggested that the upcoming by-election, due to take place next month, could be a small-scale test of whether the Prime Minister's rise to power has truly quashed the threat from the left.

"Because Keir’s seat was a target and Polanski is running in it, this gives the PM a perfect opportunity to see if the Green threat has gone with a change of leader," a party insider told The Independent.

“We are seeing people come back [from the Greens] to Labour. Under Keir, there was no doubt that we were losing lots of supporters and party members to the Greens."

They further claimed that a by-election win against Mr Polanski will prove Labour is "in good shape to fight a general election".

Meanwhile, another source said that Ed Miliband's move to impose sanctions on Israel had "nullified some of the Green appeal".

During his time as Prime Minister, Sir Keir oversaw the rise of the progressive left, with Labour voters flocking to the Greens across former heartlands - including the previously-safe Gorton and Denton seat.

Hannah Spencer, the plumber-turned-MP, stormed to victory in the widely-anticipated North West contest, taking home a 4,402 majority.

Labour fell to third place, winning around 25 per cent of the vote.

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After Mr Burnham, who resides within party's soft-left faction, entered Downing Street, strategists are hoping Camden Council leader and refugee Sagal Abdi-Wali could pull back support.

Previously, the Prime Minister has repeatedly ruled out the prospect of triggering an early election.

At the start of September, he told the BBC: "I don't think people want that."

And, more recently, a Number 10 insider said: "This is complete nonsense, the Prime Minister has ruled out an early election."

Over the weekend, Britain's leading polling guru, Sir John Curtice, warned Labour against calling a snap election because Reform UK could still become the biggest party in Parliament.

The expert said "the numbers are not there" for Andy Burnham to guarantee a Labour majority.

While Labour has seen a surge in support in the polls surge since Mr Burnham succeeded Sir Keir Starmer, Reform UK could still become the largest party in the Commons, Sir John said.

The polling guru suggested because Reform UK supporters are "geographically evenly spread", it could work to the party's advantage.