Fighting for his survival as potential leadership rivals circle, Starmer pledged to deliver more radical change to the country, an admission that he had been too timid in the face of two decades of economic stagnation and mounting social tensions.
In a speech that was light on new policies but filled with pleas to his party and personal attacks on his opponents, he said he would build closer ties with the European Union, create better job opportunities for young people and nationalise British Steel.
"I'm not going to walk away," Starmer said on Monday.
Starmer was seeking to appeal to a growing number of Labour MPs who have turned on him after his party on May 7 suffered the worst local election results for a governing party in more than three decades.
That prompted a little-known former junior minister to threaten to seek a leadership contest, a move that could force a wider leadership race if she can get the backers.
Starmer warned his centre-left Labour Party could not afford to turn on each other, confronted by the "very dangerous" populist Reform UK and left-wing Green Party, which he said wanted grievance and division: "looking not for solutions but for someone to blame".
"I know that people are frustrated by the state of Britain. Frustrated by politics, and some people are frustrated with me," he said in a speech in London.
"I know I have my doubters and I know I need to prove them wrong. And I will."
Starmer won one of the largest parliamentary majorities in modern British history in 2024 with promises to expand the economy, lower illegal immigration, and cut waiting lists in the state-run health service.
However, progress has been hampered by policy U-turns, perceptions among some voters that he is weak, and a series of political scandals, contributing to some of the lowest approval ratings of any British prime minister.
Reform UK, led by Brexit campaigner Nigel Farage, has now led national opinion polls for more than a year and at the local elections the Greens also made large advances in British cities.
Starmer sought to show he had got the message from his angry MPs, saying in his London speech that "incremental change won't cut it" and pledging to deliver more urgent action to tackle the country's many problems.
His future will depend on whether his despondent party agrees to give him time to do so.
"Part of leadership is knowing when it is time to pass on the baton," Labour MP David Smith said in a statement following the speech.
"I therefore believe that now is the time for the prime minister to set a clear timetable for his departure."
