Andy Burnham has begun his march on Westminster - and potentially to Downing Street - after winning the Makerfield by-election.

The Greater Manchester mayor was declared the constituency's new MP in the early hours of Friday, as he defeated his Reform UK rival to bring an end to his 9-year hiatus from the House of Commons.

It is Mr Burnham's stated intention to now challenge Keir Starmer for the Labour leadership - which he will be entitled to do once he formally returns to Parliament next week - with a view to replacing the embattled Prime Minister in Downing Street.

This will bring fresh civil war to Labour, with an increasingly obstinate Sir Keir having so far defied calls from his leadership rivals and disillusioned MPs to step down.

Allies of Mr Burnham used the by-election result to step up their demands for Sir Keir to agree to an 'orderly and managed' transition of power as they hope for a 'coronation'.

But those Labour MPs who remain loyal to the PM warned Mr Burnham - or other potential challengers to Sir Keir - against plunging the Government into 'chaos' and risking an early general election with a coup bid.

Turnout for the Makerfield by-election was 58.75 per cent, which is higher than the 52.4 per cent at the 2024 general election and the highest for a parliamentary by-election in nearly seven years.

The by-election contest saw weeks of frenetic campaigning, with an estimated 3,000 Labour activists having packed the streets on Thursday in an unprecedented drive to get voters to the polling stations.

In a separate by-election contest in Aberdeen South, the Tories secured a huge win over the SNP after they appeared to have successfully turned the race into a referendum on oil and gas drilling in the North Sea.

But the SNP did hold Arbroath and Broughty Ferry in a third by-election result of the night.

It has been claimed that Mr Burnham, who will now have to quit as Greater Manchester mayor after winning in Makerfield, already has the required 81 nominations from Labour MPs needed to trigger a leadership contest under the party's rules.

He intends to present the list to Sir Keir to urge the PM to step down without a contest, according to reports.

But Sir Keir is said to have amassed a war chest to fund his campaign to fight any leadership challenge and prevent a 'coronation' for Mr Burnham as Labour's new leader.

Sources said the PM has the backing of a group of private donors, with fundraising having ramped up in the last two days and total pledges running into six figures.

Former Cabinet minister Louise Haigh, the Labour MP for Sheffield Heeley who was in charge of Mr Burnham's by-election campaign, demanded Sir Keir agree to an 'orderly and managed' transition of power.

She urged the PM to 'take the time to reflect on these results, reflect on the results just six weeks ago [in the local elections] and do what’s best for both the country and the Labour Party'.

Labour grandee David Blunkett, the former home secretary, said Sir Keir and his wife Victoria should decide to leave Downing Street 'with dignity' if the desire for a change in leadership among MPs becomes 'so obvious'.

But Mike Tapp, a Home Office minister and Starmer loyalist, said it was 'really important' that Labour 'stick with' the PM to avert 'chaos' and 'churn' in Government.

'You don't change two years in, all that does is it says to the public that they can't trust that party in Government,' he told the BBC.

In a message to his fellow Labour MPs, Mr Tapp added: 'If there's a leadership contest and Keir Starmer is replaced then our opposition will have credible calls for a general election - just as we did when we called for a general election when the Tories changed their leader.

'Do they want an early general election? That's the question.'

Reform had mounted stiff opposition to Labour in Makerfield, with Nigel Farage's party intent on winning the constituency for themselves to dash Mr Burnham's long-held ambition of becoming PM.

Mr Farage and Reform's candidate, self-employed plumber Robert Kenyon, spent the last month urging local voters to 'vote Reform, stop Labour' in their efforts to derail Mr Burnham's bid for No10.

But Reform's campaign was hampered by the decision of Restore Britain, the hard-Right party set up by ex-Reform MP Rupert Lowe, to also stand in the constituency.

Sir Keir's challengers still want the embattled PM to set out a timetable for his departure from No10 himself, to avoid them having to trigger a bitter contest themselves.

Some are hoping that senior ministers will tell him at next Tuesday's Cabinet meeting that his time is up as he has become the most unpopular PM in history.

But Sir Keir remains determined to fight on and could try to shut down any discussion of his future around the Cabinet table, as he did last month despite dozens of his MPs telling him to quit.

He and his team insist he alone has the mandate to lead the country, less than two years after he won a landslide general election, and see no reason why he should give it up.

Sir Keir will not travel to Makerfield as is usual the morning after a by-election victory.

Instead the PM will speak to broadcasters on a visit elsewhere and, although he is prepared to welcome Mr Burnham's victory, he will say that a leadership contest would be a dangerous mistake.

However if any of his challengers - who also include former Health Secretary Wes Streeting - does decide to trigger one by announcing they have the requisite 81 backers, he will take part in it and so try to cling on as Labour leader and PM.

One senior source told the Daily Mail: 'He will be clear that a challenge would be wrong for the party and the country. But if there was one he is ready to fight it.

'I don't know why Andy Burnham thinks he's got the right to a coronation just because he's won a by-election.'

Another insider said: 'He feels a sense of duty and is not going to walk away.'

The source said rival camps are desperate for Cabinet ministers to move first and tell Sir Keir he has to go, so that they are not seen as disloyal by starting the race.

They also questioned Mr Burnham's sense of entitlement but predicted he and his team would soon realise the scale of the challenge facing him.

'They're on a high, thinking they've won a by-election and now they're going to take No 10,' they said.

'But the reality of coming back to Westminster is going to hit them like a ton of bricks.'

Jockeying is already under way among senior Labour figures for a place in Mr Burnham's top team if he does take over from Sir Keir.

Ed Miliband, whom the Daily Mail revealed could quit as Energy Secretary next week to trigger the coup that brings down Sir Keir, is widely expected to become Chancellor.

Louise Haigh, who was forced to quit as Transport Secretary after it emerged she had a historic conviction for fraud, has led the by-election campaign and could be made Health Secretary.

Anneliese Midgley, who was once an adviser to Sir Keir, would be Chief Whip in Mr Burnham's administration and his policing chief Kate Green has been tipped as his Chief of Staff.

Rachel Reeves, who is keen to remain Chancellor even in a new administration, warned leadership contenders not to risk changing her fiscal rules.

She told a conference: 'We got elected on the promise to return stability to the economy, and anyone who wanted to deviate from that will not be fulfilling the manifesto commitments.'

Mr Streeting is reported to have met Mr Burnham while campaigning in Makerfield on Monday, though a source said 'no deals were done' as a result of the meeting. 

Even if Mr Burnham does become PM, many expect his honeymoon period to be even shorter than the one Sir Keir enjoyed.

He will face calls to hold a general election as he will not be seen as having a mandate, while he will lack the money or tax-raising powers to implement the Left-wing policies backbenchers want, such as nationalising utilities.

'This PLP [Parliamentary Labour Party] is not scared of speaking out,' a source said.

Despite Sir Keir's insistence that he is not going anywhere, some still expect he will be forced to step down soon.

Resignations could take place in the coming days to pile further pressure on him, amid reports that 'trigger happy' ministers have been told to hold fire over the weekend.

Others want senior figures to use Tuesday's Cabinet meeting to talk the PM into resigning.

One source said: 'Over the weekend people who say privately Keir should go need to start saying that to him, or publicly.

'The fact is that he doesn't have the support of most of his Cabinet, his frontbench, the PLP or most of the country.

'The best way forward is to set out a timetable for future-facing candidates to debate ideas, rather than a bitter contest where it's a blame game for everything that's happened for the past two years.

'Keir will go either way but this way the agency is with him. Lots of people are hoping he will see sense.'

They added: 'Keir banned discussion of his leadership at Cabinet last month. This time they've got to insist and make their views known.'

The Makerfield contest was triggered by the resignation of Josh Simons as the local MP, who said he wanted to give Mr Burnham a chance to return to Westminster to challenge Sir Keir because Labour was 'imploding'.

Mr Simons gave up the constituency he won for Labour with a 5,000-vote majority at the 2024 general election in the wake of his party's meltdown following disastrous results in elections in Scotland, Wales and English councils in May.

'Labour needs to change and the whole Government needs to change,' he said last month.

Mr Simons has been touted for a top role in Downing Street, should Mr Burnham be successful in his bid to take over from Sir Keir.