Andy Burnham will use the upcoming by-election caused by Sir Keir Starmer’s resignation as an MP as a “test case” to decide whether to call an early general election, opposition parties believe.

Voters in Holborn and St Pancras in central London will choose a new MP on October 8 – with Green Party leader Zack Polanski standing, and hoping to cause an upset in a safe Labour seat.

This makes it the first major electoral test for Andy Burnham since he became Prime Minister, and Reform UK insiders believe the outcome will determine whether Burnham gambles on a snap general election.

A senior Reform UK source said that if the Labour candidate Sagal Abdil-Wali, the leader of Camden Council, proves Polanski is a “dud” by beating him “convincingly”, the Prime Minister could trigger an election before Christmas.

‘We’re not ready,’ Reform source says

Doing so would allow Burnham to exploit splits on the right and Reform’s current lack of preparedness for a general election, they explained.

“We’re not ready,” the source said.

A senior Conservative source agreed that if Labour won “handily” in Holborn and St Pancras it would be a “spur” to go early, while a Liberal Democrat MP said winning the seat would be a prerequisite for an election which they now expect to take place in May.

The Prime Minister has repeatedly ruled out holding an election before the end of the current term in 2029. But speculation in Westminster is nevertheless rife that he could be tempted to go early.

Observers believe Burnham could be swayed if Labour are comfortably ahead in the polls because the Tories are yet to recover, and the Greens’ early momentum under Polanski has stalled since Burnham became PM, while Reform have lost their polling lead.

In addition, a worsening economic picture and Burnham’s room to manoeuvre limited by pledges in the Labour manifesto, along with the PM not holding his own mandate for change, means the Prime Minister could face a now or never moment by May, they believe.

While Holborn and St Pancras has been a safe Labour seat since its creation in 1983, its metropolitan profile in the heart of London makes it a prime Green target. The Greens gained 10 seats on the council – bringing them to a total of 11 – in May’s local elections, although Labour remained the largest party.

The Greens also achieved a decent showing in the constituency in 2024, coming third and beating the Tories, Reform and the Lib Dems.

With the Greens now polling at about 11 per cent nationally compared with the 6 per cent it achieved in 2024, the party hopes that by putting Polanski on the ballot paper and flooding the seat with activists a historic victory is within its grasp.

‘Waiting is death’ for the Prime Minister

Although the contest is almost certain to be a two-horse race between Labour and the Greens, senior figures in the other opposition parties think it will play a crucial role in determining whether Burnham allows the Parliament to run its course to 2029 or opts for an early election.

The senior Reform source said that if Labour “smashed” Polanski with a comfortable victory, Burnham would conclude that he is successfully reuniting the progressive vote and would be emboldened to go early.

Doing so would make the most of continuing divisions on the right, they said, with the right-of-centre vote currently fractured between the Tories, Reform and Rupert Lowe’s Restore Britain.

It would also catch Reform leader Nigel Farage on the hop before he had selected candidates for seats or deployed the £72m war chest he has been handed by two crypto-billionaires, they said.

The source added that Burnham would want to push for his own mandate before the cost of living situation deteriorates in the wake of higher inflation and interest rates.

With the Ofgem energy price cap potentially breaching £2,000 a year in January because of the ongoing US-Iran war, the source predicted that Burnham would call an election before Christmas if he wins big. For the Prime Minister, “waiting is death”, they said.

A second Reform insider said: “The Green Party does not know what’s just about to hit it in Holborn and St Pancras.

“The Labour Party when it is in this sort of mood is the most vicious, the most professional, the scariest campaign machine you’ll ever see. They’re bloody effective.

“If Zack fails that might put a couple of points on Labour, and that might make [Burnham] think ‘you know what, it might be worth trying [an early election]’.

“The Greens are going to get an absolute mullering from the Labour machine and I do think this could fatally damage the Green Party after a year of success.”

‘A highly risky option’ but Burnham is a ‘risk taker’

But the insider added: “I still think it’s a highly risky option… whatever happens, Labour is going to lose 100 seats. That’s 100 people who are not going to be overly keen on the idea, including some senior ministers.”

The senior Tory source said that if Polanski loses next month the Green leader would be “severely weakened”.

“[Burnham’s] whole thing is to consolidate the left,” they said. “If he continues to constrict the Green vote then it will tempt him into an early election whilst the right remains split.

“There’s no doubt that beating the Greens quite handily will give them a bit of a spur.”

The source said that “backloaded” spending cuts pencilled in by the previous chancellor Rachel Reeves for the end of the parliament, along with higher inflation, interest rates and energy bills, all pointed to an early election.

However, they said Burnham would be weighing the risks – most obviously the near certainty that even if Labour won it would go backwards on its current Commons strength of 403 seats.

“No way you cut it can you see him retaining 400 seats,” they said. “You instantly become a loser because you will have lost seats, even if you win.”

The Lib Dem MP said the result in Holborn and St Pancras would be pivotal in deciding the election date. “If he loses to the Greens… then all of the Labour MPs who have Greens in second place [in their seats] will be lobbying him [not to go early],” they said.

The MP said they thought Burnham would most likely call an election for next May, because holding one before Christmas would be “too tight”.

“He’s a risk taker,” they said. “Makerfield was a huge risk. It often happens when people take risks and they succeed, then they get cocky.”

The MP also agreed that Burnham would look to strike while the right is divided.

“What you want to do is go to the polls while the situation is like it is now where Reform and the Tories are kind of neck and neck,” they said. “Who’s the tactical vote on the right? Really not clear. Whereas Labour and the Lib Dems have sorted it out among ourselves – we know exactly which seats are theirs and ours.”

Labour has consistently denied that there is going to be an early election. In an interview last month Burnham said: “I’m going to rule it out… I don’t think people want it.”

On Thursday, a Downing Street source told The i Paper: “It’s not going to happen.”