Speaking outside 10 Downing Street, Starmer said he accepted the verdict of his party.
Keir Starmer (right) with his wife Victoria outside 10 Downing Street in London, after a speech on Monday in which he announced his resignation as Britain's prime minister. Photo: EPA/NEIL HALL
"The question my party is asking now is whether I am best placed to lead us into the next general election," he said.
"I have heard the answer from my parliamentary party to that question, and I accept that answer with good grace," he added.
A new Labour leader is expected to be in place by the time Parliament returns on September 1.
Starmer said nominations for the leadership contest would open on July 9 and close before Parliament begins its summer recess on July 16.
If more than one candidate qualifies, the contest will be completed before lawmakers return in September, the Reuters news agency reported.
Under Labour Party rules, candidates must secure the support of at least 20 percent of Labour members of Parliament and meet thresholds for backing from local party organisations and affiliated groups, including trade unions, according to Reuters.
If more than one candidate qualifies, the winner will be chosen in a ballot of Labour Party members and affiliates and will automatically become prime minister.
If only one candidate secures the required support, that person will be elected unopposed.
Starmer came to power after leading Labour to a landslide victory in the 2024 general election, ending years of Conservative rule.
British Prime Minister Keir Starmer. Photo: Number 10, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Once praised for promising stability and pragmatism after a period of political turmoil, he later faced growing criticism from voters and members of his own party over policy reversals and what critics saw as a lack of a clear long-term vision.
Poor results in local elections in May intensified pressure on him to step aside.
Photo: EPA/TOLGA AKMEN
Andy Burnham, the mayor of Greater Manchester and a longtime political rival of Starmer, is widely seen as a leading contender to succeed him.
Supporters view Burnham as one of Labour's strongest figures in countering the growing popularity of Reform UK, led by veteran Brexit campaigner Nigel Farage.
British Labour Party politician Andy Burnham. Photo: EPA/Tolga Akmen
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Source: Reuters, IAR, PAP