Andy Burnham 'working on plans for first 100 days in No10' – before he's even been elected to Parliament

Plans are in their infancy, according to three insiders who have worked with Mr Burnham, but will likely address England's struggling social care system.

Speaking to Politico, one insider said the plans will likely include proposals to lower energy costs for UK households.

The plans are an indication of the confidence Mr Burnham's supporters have that he will become the next Labour leader.

Staff are also having to revisit historical comments made by Mr Burnham. Four years ago, the Mayor of Greater Manchester argued it is a "minority view" female-only toilets should be exclusive for biological women.

Mr Burnham's team has ruled out changing existing fiscal rules, despite him saying in the past he could do so to increase defence spending.

Meanwhile, Mr Burnham promised he would not "re-run" the Brexit debate, even though he previously said he would like to see the UK rejoin the EU during his lifetime.

The speed at which Labour's local election losses on May 7 destabilised Sir Keir Starmer's leadership has thrust Mr Burnham into pole position to succeed him.

The 56-year-old mayor's allies are now simultaneously running a by-election campaign against Reform UK, who announced their candidate, local plumber Robert Kenyon, yesterday, as well as drawing up a national policy for Mr Burnham's potential succession to Government.

One source, who has worked closely with the Greater Manchester mayor, said there was concern not enough work had been done on Government-level policy, though they believed there was still time to develop something more comprehensive.

They warned: "If you don't get this right, you end up with Nigel Farage as Prime Minister."

Labour formally selected Mr Burnham on Tuesday as its candidate for a June parliamentary by-election in Makerfield, a collection of towns and villages near Wigan in the north west of England.

If he wins the seat and becomes an MP, the former Health Secretary under Gordon Brown, will be eligible to trigger a leadership challenge against the Prime Minister, which polling suggests he would win.

A YouGov survey found Mr Burnham would secure 59 per cent of the Labour membership vote against 37 per cent for Sir Keir in a straight contest between the two.

The polling also showed he would comfortably defeat former health secretary Wes Streeting in a leadership race, winning 80 per cent of the vote to Mr Streeting's 10 per cent.

Whilst stopping short of explicitly calling for Sir Keir to go, Mr Burnham said in a speech on Monday: "A vote for me will be a vote to change Labour, because Labour needs to change if we're to regain people's trust."

He has set out a series of policy positions, including bringing privatised energy and water firms "back under public control", reclaiming state control of housing and reforming technical education.

On social care, Mr Burnham's allies are said to be frustrated by the pace of existing government work on the issue.

One source said there was a strong sense within his team the matter was "just too urgent" to wait for the outcome of Baroness Casey's review, which is not due to report until 2028.

The Makerfield by-election is likely to take place on June 18, however, this date is yet to be confirmed.

Alongside Mr Burnham and Mr Kenyon, Restore Britain has named businesswoman Rebecca Shepherd as its candidate, who was the first to be officially announced.

The Official Monster Raving Loony Party will also be fielding a candidate, with leader and co-founder Alan "Howlin" Laud Hope confirmed to stand.

The party said in a statement that their "glorious leader" would be running against Mr Burnham in the contest.

Other party candidates are expected to be announced in the coming days.