Washington DC - President Donald Trump on Sunday said he wished UK Prime Minister Keir Starmer well amid reports he plans to resign.

"Keir Starmer will resign as Prime Minister of The United Kingdom," Trump wrote on Truth Social in a post that gave no evidence he had inside information. "I wish him well."

His post came as British media reported that the embattled Labour Party leader will resign in a matter of days.

Trump has slammed Starmer for not supporting the US-Israeli war against Iran, and in his post on Sunday, the president repeated criticism of the British premier on two issues: borders and fossil fuels.

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"He failed badly on two very important subjects- IMMIGRATION AND ENERGY (OPEN NORTH SEA OIL!)," Trump wrote.

A senior minister said Starmer was reflecting this weekend on the "political realities" that face him.

The speculation follows veteran Labour politician and rival Andy Burnham's victory in a by-election on Friday, which paves the way for the 56-year-old to challenge the beleaguered Starmer to be party leader and prime minister.

Any Labour leadership candidate must be a member of parliament.

If Starmer does leave office this year, Britain will get its seventh prime minister in a decade – an unprecedented rate of churn in its modern history.