Sir Keir Starmer met Taylor Swift when he attended her concert at Wembley Stadium, Sky News understands.
It has emerged the prime minister and his family spoke to the pop star and her mum for 10 minutes and discussed the Southport stabbings.
The revelation comes amid questions over government intervention in talks over the security for the concerts after Swift was given a taxpayer-funded police escortdespite reservations from the Met.
It had not been clear previously if Sir Keir and Swift had met at the gig.
There was no discussion about the provision of security for the artist, which Downing Street said was an independent operational matter for police, it is understood.
Sir Keir and his family were given free tickets to the concert on 20 August, which were declared as required, and have since been paid back.
The meeting came after the superstar cancelled gigs on her Eras Tour in Austria due to a terror threat, and the mass stabbing in Southport at a Taylor Swift-themed dance class, when three young girls were killed.
Sir Keir was invited to the August show by Universal Music, which is based in his constituency of Holborn and St Pancras.
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1:40
Questions over Taylor Swift motorcade
Further questions were raised after a report in The Sun alleged Home Secretary Yvette Cooper and London Mayor Sadiq Khan spoke to the Metropolitan Police to encourage them to give the megastar a “VVIP escort”, when she returned to the capital to complete the European leg of her sell-out tour in August.
The Met was reportedly reluctant to sign it off as a blue-light escort is typically reserved for senior members of the Royal Family and high-level politicians, because it comes at huge expense to the taxpayer, the newspaper reported.
Swift’s mother Andrea, who is also her manager, apparently threatened to pull her daughter’s three shows if the police convoy was not provided.
The government has denied that senior Labour figures were given tickets to the shows in exchange for police protection, stressing the Met makes decisions independently from politicians.
Last week, deputy prime minister Angela Rayner said it was down to the fact Swift’s concerts in Vienna had been cancelled due to a foiled terror attack, which was intended to kill tens of thousands of fans.
“We needed to make sure that that person was safe. And it was a policing matter, not an issue for politicians. It was the police that make the decision,” she said.
She added that police provided the security to ensure Swift could continue with the concerts “which brought in huge amounts of investment of money into our economy, including those small businesses that need that support”.
Culture Secretary Lisa Nandy told Sky News “you would expect” the home secretary and the mayor to be involved in a conversation “where there is a security risk”, such as after the Vienna bomb plot.
“I really utterly reject that there’s been any kind of wrongdoing or undue influence in this case,” she said.
Cabinet ministers are writing to the prime minister in an urgent attempt to soften some of the spending cuts being demanded ahead of the budget.
The main measures for the budget have to be decided by the end of Wednesday and sent to the Office for Budget Responsibility before the Treasury shuts up for the evening.
Sky News can confirm letters from cabinet ministers complaining about the budget have gone over the head of Chancellor Rachel Reeves to Sir Keir Starmer and Number 10.
Some of these letters are believed to have gone in over the last couple of days.
After today, only small changes can ordinarily be made to the budget, typically around involving items of tens or hundreds of millions rather than billions. The existence of the letters was first reported by Bloomberg.
Some cabinet ministers are deeply concerned about the scale of the cuts being demanded in some areas to fund pay rises and spending increases elsewhere.
It is understood that Number 10 has received complaints from four government departments: the Ministry of Housing, Communities and Local Government, the Department for Transport, the Department for Environment, Food and Rural Affairs, and the Ministry of Justice.
Not all of these complaints were made through the medium of a letter.
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It is understood the call for an intervention by Number 10 has helped soften a small portion of the cuts for some cabinet ministers’ departments.
But largely the shape of the budget, which is for the most part wrapped up tonight, has remained unchanged – and so have the nerves about how it will land in two weeks.
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3:05
Could chancellor ‘find’ more money?
At the weekend, Sky News revealed one cabinet minister said: “The briefing doesn’t match the reality. It’s pain this year, and pain next year. We’re simply going to be digging a hole which we end up filling in later in the year.”
Ms Reeves will loosen the borrowing rules in the budget in order to give herself more room for spending, along with raising up to £40bn in tax rises and welfare cuts in order to relieve pressure on budgets.
But the cost of significant public sector pay rises, which are still going to have to be found from within departmental budgets, mean cuts this year and next are still being demanded by the Treasury.
Some ministers believe the cuts are unsustainable, while some have pointed to the first round of spending reductions announced before the summer – including the winter fuel allowance changes – as evidence more input from Number 10 is needed ahead of the budget and spending review on 30 October.
One government source told Sky News: “The whole of Number 10 is focused on the budget at the moment and there isn’t much bandwidth for anything else.”
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Government figures insist that letters voicing concern are routinely sent at this point in a spending review process and that this is all normal.
Both Sir Keir and Ms Reeves are out of the country next week, the week before the budget. However, only smaller changes can typically be made after today to the shape of it.