The sister of murdered MP Jo Cox is to be Labour’s candidate in the Batley and Spen by-election, a poll that could determine whether Sir Keir Starmer survives as party leader.

Kim Leadbeater, 44, who only joined the Labour Party recently, will defend the seat once held by Ms Cox.

Until recently it was held by former Coronation Street actress Tracy Brabin who quit as MP after being elected to the new post of West Yorkshire metro mayor.

In a crucial intervention this weekend, Labour’s “King of the North” Andy Burnham, mayor of Greater Manchester and favourite to succeed Sir Keir as party leader, backed Ms Leadbeater, claiming she was “not a typical politician” and “exactly the kind of voice Labour needs”.

Ms Brabin had a majority of 3,525 over the Conservatives at the December 2019 general election, but after the Tories’ spectacular victory in the Hartlepool by-election they are odds-on favourites for another “Red Wall” triumph.

That would almost certainly trigger moves by Sir Keir’s left-wing critics in the party and trade unions to oust him, with Jeremy Corbyn’s close ally Diane Abbott already predicting a Labour defeat in the by-election “must surely be curtains” for Sir Keir.

Ms Leadbeater is an ambassador for the Jo Cox Foundation, leader of the More in Common Batley and Spen community volunteer group and was awarded an MBE in the New Year Honours for her work on tackling social isolation during the coronavirus pandemic.

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The Conservatives have selected Ryan Stephenson, a Leeds city councillor and chairman of the West Yorkshire Conservatives, as their candidate.

“My campaign will focus on how Batley and Spen can build back better as we emerge from the COVID-19 pandemic,” he said.

A date for the by-election has not yet been set, but it is thought Labour is considering holding it on 22 July, the day the Commons breaks up for the summer recess.

That would make it harder for Sir Keir’s enemies in the party to move against him.

Ms Cox was elected in 2015 but was murdered outside a constituency surgery by a far-right extremist a week before the 2016 EU referendum.

The seat, between Bradford, Leeds, Huddersfield and Wakefield, was held by the Tories from 1983-97 but has been Labour since.

But the party faces four formidable obstacles in defending the seat, which voted 59.6% Leave – lower than in Hartlepool, where 69.6% voted for Brexit – in the referendum and where its 2019 majority was almost identical to that in Hartlepool:

• The Tories “Red Wall” surge, with victory in Hartlepool and major gains in town hall elections in the north of England, has given Boris Johnson and the Conservatives the momentum to pull off another shock by-election victory

• A pro-Brexit independent, ex-rugby league professional Paul Halloran, polled 6,432 votes in the 2019 election and may stand again

• George Galloway, the firebrand left-wing former Labour MP, says his Workers Party GB will stand. If he is the candidate, he is likely to highlight violence in Gaza to woo Muslim voters and he won a by-election in nearby Bradford West, with one of the largest Muslim populations in the country, in 2012.

• A row over the suspension of a Batley Grammar School teacher who showed pupils a cartoon of the Prophet Mohammed, prompting days of protests in March, could become a big issue in the campaign and create difficulties for Labour.

Sir Keir is already under enormous pressure from inside his party after Labour’s humiliation in Hartlepool and his bungled response, which included a botched attempt to side-line his deputy, Angela Rayner, and then a retreat in which he ended up promoting her.

It is claimed one blunder by the Labour leadership was not realising Ms Brabin would have to quit as an MP because the metro mayor is also police and crime commissioner, which meant she could not continue in the Commons.

Sir Keir is also feeling the heat from Mr Burnham, who last weekend said in an interview about his 2015 leadership bid: “I don’t think we’d have lost as many northern seats had I won.”

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Hartlepool turns blue for first time

Mr Burnham told The Observer he would “not be returning to Westminster any time soon” and then added: “But if there comes a point where it is clear to me that the Labour Party, having not thought me right twice, suddenly thinks ‘Well actually you probably are now, because of the way the world has changed’, then, as I say, I will put myself forward to lead the Labour party.”

That has prompted left-wing Labour MPs to pile in with their support, with Ms Abbott writing in The Guardian about Sir Keir: “If Labour loses again, it must surely be curtains for him. And then it may be that Andy Burnham’s time will have come.”

Leading Corbyn supporter Jon Trickett then tweeted: “It will go nuclear after Batley.”

And Rebecca Long-Bailey, defeated by Sir Keir last year, said: “There are going to have to be serious discussions if there is a loss in Batley and Spen.”

From the Blairite wing of the party, former minister Lord Adonis wrote after the Hartlepool defeat: “Unfortunately, he turns out to be a transitional figure – a nice man and a good human rights lawyer, but without political skills or antennae at the highest level.”

And last week the top historian and political commentator Sir Anthony Seldon wrote last week: “Keir Starmer is a capable man. But he is no party leader. He should stand down now in the interests of Labour.”