Prime Minister Keir Starmer said on Monday he would resign, promising to ensure an orderly transfer of power to a new leader by September at the latest to try to avoid political instability as Britain prepares for its seventh leader in 10 years.

Less than two years after he won a landslide election victory that promised to end chaos in British politics, Starmer said he would support whoever replaced him.

"I was told time and time again that my party was finished, that we were consigned to history, that a majority at the general election, let alone a landslide majority, was impossible, but we proved those people wrong, because we changed our party, ripping out the poison of antisemitism, restoring trust on the economy, defence, and national security, and becoming a party that once again stood proudly with, not against, our national flag," Starmer said in his speech.

The threat to Starmer, which had been building for months, increased sharply on Friday when Andy Burnham, the Greater Manchester mayor, decisively won a parliamentary election to return to Westminster, beating a candidate from Nigel Farage's Reform UK party, which has led national opinion polls for more than a year.

That victory gave hope to Labour lawmakers that Burnham, a career politician known for his communication skills, could transform the fortunes of a party that has lost support under Starmer, whose popularity ratings have sunk to the lowest for any British leader.

Burnham said on Monday he would put himself forward to enter the contest to replace Starmer, hours after the British prime minister announced he was resigning.

Next prime minister to become Britain's seventh since Brexit

Whoever replaces Starmer will become Britain's seventh prime minister since the Brexit vote to leave the European Union, which took place 10 years ago this week.

That level of turnover - the highest in Britain in nearly two centuries - underlines the struggle of maintaining the support of voters angry at successive failures to improve living standards, public services, and tackle illegal immigration.

The political advisory group Eurasia had said the best outcome could be for Starmer to say he will step down in September, enabling him to attend a UK-European Union reset summit in July and give Burnham time to prepare for government.