LONDON (AP) — U.K. Prime Minister Keir Starmer is facing a career-defining decision: step down or fight a possible challenge from Labour Party rival Andy Burnham.

Starmer has publicly vowed to stay in office, but pressure is building as more and more Labour Party colleagues conclude that his time is up. Expectation is growing that he will announce a timetable for his resignation as soon as Monday. That's the day Burnham will be sworn in as a lawmaker in the House of Commons after winning a special election last week.

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Business Secretary Peter Kyle said Sunday that Starmer is "making time to reflect on the political realities, challenges and opportunities that he finds himself in."

"I know he is a prime minister who always puts his country first," Kyle told the BBC, though he said that reports that Starmer will resign are "speculation."

Starmer is spending the weekend at Chequers, the country mansion used by prime ministers, with his family. He gave no public hint about his decision, but sent a Father's Day message on social media.

"Being a dad is my greatest joy. Today, I'm thinking about my dad, and the father I am to my children because of him," he wrote on X.

U.S. President Donald Trump weighed in even before an announcement, linking Starmer's potential exit to two of his recurring bugbears: immigration and renewable energy.

"Keir Starmer will resign as Prime Minister of The United Kingdom. He failed badly on two very important subjects- IMMIGRATION AND ENERGY (OPEN NORTH SEA OIL!). I wish him well! President DJT," Trump posted on his Truth Social network.

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It was unclear whether Trump was responding to media reports about Starmer's plans. The two leaders haven't spoken over the weekend.

Starmer's initially warm relationship with the president has soured in recent months over issues including the Iran war, which the U.K. didn't join.

If Starmer quits, he will be the sixth prime minister to leave office in the past 10 years, an extraordinary rate of churn for the United Kingdom.

Discontent with the prime minister has been building for months, with Labour lawmakers desperate to reverse the government's decline in popularity since Starmer led the center-left party to a landslide election victory in July 2024.

He has struggled to deliver promised economic growth, repair tattered public services and ease the cost of living, and has been hamstrung by repeated missteps, including his decision to appoint Peter Mandelson, a scandal-tarnished friend of Jeffrey Epstein, as the U.K. ambassador to the United States.

Labour is losing liberal voters to the growing Green Party and facing a rising Reform UK, the Nigel Farage -led anti-immigration party that consistently leads in nationwide opinion polls.

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Burnham, until this week the popular mayor of Greater Manchester, decisively won the seat of Makerfield in northwestern England in a special election held Thursday. He took almost 55% of the 45,510 votes cast, over 9,000 more than the Reform UK runner-up.

Now that Burnham is becoming a lawmaker, he's in a position to challenge Starmer for leadership of the Labour Party. Burnham's acceptance speech left no doubt that he wants to lead both the party and the country.

"Everyone knows that politics isn't working," he said. "Everyone can feel that the country isn't where it should be. Tonight could, just could, be the turning point."

It's unclear whether Burnham would face a coronation or a challenge, if Starmer steps aside. Wes Streeting, who resigned as health secretary last month to protest Starmer's leadership, has said that he will run in a contest if there is one.

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Starmer congratulated Burnham on Friday, but insisted that he would fight any attempt to oust him.

"I will run, I will stand," if there is a Labour leadership contest, Starmer said. "I've said repeatedly I'm not going to walk away from that."

But Charlie Falconer, a senior Labour member of the House of Lords, said Saturday that Starmer has "absolutely no authority" left.

"There should be an agreed transition process in which Andy and Keir cooperate as to when the handover should take place," he told the BBC.