A Norwegian energy giant has threatened to pull UK investment if Andy Burnham blocks two of the North Sea's largest untapped oil and gas fields.
Anders Opedal, the chief executive of Equinor, told the Energy Intelligence Forum in London it would be a "major setback" if officials refused permission to drill at the Rosebank and Jackdaw sites.
Mr Opedal said: "We would then ask, 'Is the UK investable?'"
His remarks came after the Government delayed a decision on the Jackdaw gas field until after a key by-election this week, where Zack Polanski, the Green Party leader, is seeking to win Sir Keir Starmer's, the former prime minister's, old seat .
The oil industry is also awaiting a decision on Rosebank, the UK's largest untapped oil reserve.
Rosebank and Jackdaw are both being developed by Adura , a joint venture controlled by oil giant Shell and Equinor.
Mr Opedal said: "Not approving Rosebank would be contradictory to energy security, to job creation."
A decision had been expected in September, but The Telegraph revealed it had been pushed back until after the by-election in Holborn and St Pancras on Thursday.
The previous Conservative government originally approved Rosebank in 2023 and Jackdaw in 2022. However, the projects have been beset by delays and legal challenges, with environmental campaigners managing to overturn the original approvals, forcing fresh applications in 2025.
Mr Opedal said: "The Keir Starmer government said the UK was open for business. We've applied for consent twice ... Adura has done everything it has been asked to do. The UK has one of the most rigorous approval processes in the world."
Mr Opedal said he anticipated the project would be approved.
Labour's 2024 manifesto said it would issue no new North Sea licences, but that it would honour existing ones.
It comes as experts warn that Britain runs the risk of soaring energy prices and potential fuel shortages this winter, as Donald Trump's war in Iran upends international energy markets .
Jackdaw could begin production this winter if approved, as much of its infrastructure is already in place.
The gas field is 150 miles east of Aberdeen. It could produce enough energy to heat more than 1.4 million homes at peak production.
Rosebank, near the Shetlands, meanwhile, is the UK's last major undeveloped oil site. It is believed to contain up to 300-500 million barrels of oil and some gas.
At their peak, Rosebank and Jackdaw could provide 10pc of the UK's oil and gas output.
However, a significant number of Labour MPs oppose approving the two projects, while Mr Burnham also risks an embarrassing by-election defeat by Mr Polanski's Greens, who have campaigned against new drilling.
The final decision on the Jackdaw and Rosebank applications lies with Miatta Fahnbulleh, the Energy Secretary.
She has previously declared that drilling in the North Sea is "irresponsible and short-sighted", warning that Britain needs to reach net zero "fast".
The row about Rosebank reignited in recent days after Mr Polanski warned Ed Miliband, the Foreign Secretary, that approving it risks breaching his own sanctions on illegal West Bank settlements.
The Green Party leader raised concerns about Israeli-controlled Ithaca Energy, which holds a 20pc stake in Rosebank.
It is controlled by Israeli energy giant Delek Group, which the UN describes as supporting "the maintenance and existence" of settlements in occupied Palestinian territories. Delek has said it plans to challenge the UN designation.
