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The ex-Prime Minister insisted he would serve a full-term as the MP for Holborn & St Pancras

Sir Keir Starmer has resigned as the MP for Holborn & St Pancras and triggered a by-election headache for Andy Burnham.

The ex-Prime Minister, who vowed to remain as an MP until the next general election, made the final U-turn of his political career shortly before Mr Burnham is due to face his first Commons showdown since entering No10.

In a 372-word resignation letter, Sir Keir wrote: "Over the summer, I have had time to reflect and consider my future.

"I have decided that now is the right time to step down as MP for Holborn & St Pancras and hand the baton over to a successor.

"It's been an honour and privilege to represent the best constituency in the country since taking over from the great Frank Dobson 11 years ago.

"But it is now time to step aside and focus on other issues: international affairs including defence, security, trade and technology in a fast-changing world."

There had been speculation that the ex-Prime Minister was being lined up for a top job in Brussels.

Nato’s secretary-general position, which is currently occupied by ex-Dutch Prime Minister Mark Rutte, could potentially become vacant in 2028.

A series of informal meetings will need to take place before a final decision is taken, with the consent of all 32 member states required.

Sir Keir travelled more and further than any other British leader in official history, including almost double the distance that Sir Tony Blair covered in the same period in office.

The 63-year-old announced he is standing down as an MP just six weeks after resigning as Prime Minister on July 20.

He added: "I want to thank my CLP team who have always stood firmly by me, and given me the support I needed as I stood for Leader of the Labour Party and then as I took my Party from political, moral and financial ruin to an historic landslide election win in 2024 and two years as Prime Minister when we stabilised the economy, rebuilt our public services, invested record amounts in our defence and security, began lifting hundreds of thousands of children out of poverty and transformed Britain's reputation with international partners.

"I want to thank my constituency staff who, throughout all those 11 years, have worked tirelessly on casework and helped me do my utmost to serve the people of Holborn and St Pancras.

"I could not have done this job without them. There will now be a process to appoint my successor ahead of a by-election.

"Whoever is lucky enough to become the Labour candidate here will have my full support and backing and I will continue to give my full support to the Prime Minister and this Labour Government."

Sir Keir was first elected as the MP for Holborn & St Pancras in 2015, with his 30,509 vote majority in 2017 eventually being whittled down to 11,572 in 2024.

Despite Holborn & St Pancras being a safe Labour seat since its inception in 1983, the Green Party is widely considered the main challenger in the seat.

Green Party leader Zack Polanski, who sits as a London Assembly Member, could potentially stand in Holborn & St Pancras.

"Zack has always made it clear he would like to be a London MP," a Green Party spokesman said.

The 2026 Local Elections saw the Green Party surge from just one to 11 seats in Camden, while Labour dropped from 45 to just 30.

The Greens had claimed victory at Labour's expense when Hannah Spencer won the Gorton & Denton by-election in February.

However, a recent bounce in the polls following Sir Keir's resignation appears to have helped Labour open up a considerable lead over the Green Party.

Sir Keir's resignation means he follows in the footsteps of Sir Tony Blair and David Cameron, who both opted to trigger snap by-elections after quitting as MPs in 2007 and 2016.