The MP for Morecambe and Lunesdale has joined a growing number of Labour figures to call for Sir Keir Starmer to resign.
Lizzi Collinge said on Tuesday that the Prime Minister had "lost the confidence of the country".
The BBC reported on Tuesday morning that 80 Labour MPs, just under 20 per cent of the party's total of 403 in the Commons, have called on Starmer to leave his post, or set out a timetable for his resignation.
This follows the local election results last Thursday, where Labour was down 1,498 councilors from the last local election in England, and lost power in the Welsh Parliament for the first time in more than a century.
Any leadership challenger would need the support of 20 per cent of Labour MPs - 81 people - to trigger a contest.
"I have been taking time over the past week to reflect on the local election results elsewhere in the country, to reflect on our first two years in Government, and of course to speak with my constituents," said Ms Collinge in a statement on Tuesday.
"I am immensely proud of the progressive and important legislation we have passed, from the Employment Rights Act to the Water (Special Measures) Act.
"I am also proud of the clear and concerted action we have taken on the most pressing issues of our time: economic stability, clean energy and climate change, national security, and improving the NHS.
"We had a very difficult inheritance from 14 years of destructive Conservative government, and any Prime Minister would have struggled to solve the problems our country faces in two years.
"I also recognise that the Prime Minister has led us through some extremely difficult international situations with skill and I thank him for that.
"However, it is clear to me that the Prime Minister has now lost the confidence of the country. And as an MP, my first duty is to my constituents and to my country. It is for this reason that I am now calling on the Prime Minister to step back and arrange an orderly transition to new leadership.
"I believe that Labour values and policies are still the only way to improve our country and we need a Prime Minister who can show the country what progress we have made and can still make.
"Labour don’t offer simplistic slogans, we offer real solutions. Governing a country requires compromise and hard choices, and we're the only party who faces up to that.
"We also face some opposition that simply operates outside the norms of decency and honesty. I have seen this locally and nationally. We need to fight to steer our country away from the terrible future they want, a future riven with division and with policies based on prejudice and ideology, not facts, evidence or positive values.
"I love our country and I love the people in it. I will keep on working to make life better for every person in Morecambe and Lunesdale, and for every person in the UK."
Beyond Radio has contacted Cat Smith, Labour MP for Lancaster & Wyre, about the issue. Ms Smith said she had no comment to make at this stage.
The Prime Minister held a cabinet meeting on Tuesday morning, and afterwards Business Secretary Peter Kyle backed him, saying Starmer has shown "steadfast leadership”.
But Miatta Fahnbulleh, Minister for Devolution, Faith and Communities, has resigned, saying: "We have not acted with the vision, pace and ambition that our mandate for change demands of us."
Sir Keir Starmer was appointed Prime Minister after Labour won a landslide victory at the 2024 General Election.
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