What many in Westminster expect is about to happen is almost completely unprecedented in British political history.Andy Burnham, a two-time Labour leadership loser, looks set to secure the keys to No10 just 28 days after his long-awaited return to the House of Commons. The King of the North, who is not a new face in Westminster, is the only candidate to publicly declare he will hand in his nomination papers on July 9.Ex-Armed Forces Minister Al Carns is still considering a run for the top job, albeit the silver-haired Scotsman’s ultimatum to Mr Burnham also came with a nod about serving in his Cabinet. But what happens if Mr Carns opts against throwing his hat into the ring?GB News breaks down why Mr Burnham could become Prime Minister in just a few weeks’ time. Labour’s ruling body set out the timetable for electing a new party leader earlier this week.The timetable requires wannabe leadership candidates to receive the backing of 81 Labour MPs, around 20 per cent of Sir Keir Starmer’s current Commons contingent. Labour’s ruling body set out the timetable for electing a new party leader earlier this week | GETTYMr Burnham is expected to hoover up around 200 MPs ahead of the first crunch date, with Chancellor Rachel Reeves emerging as a perhaps somewhat surprising supporter. The King of the North could become Labour leader on July 17 but precedent means Mr Burnham cannot become Prime Minister on the same day.He will instead likely need to wait until July 20 to secure the keys to No10, the day after the World Cup final. The ex-Greater Manchester Mayor appears to have benefited from a lack of support for other potential rivals.Ex-Health Secretary Wes Streeting announced earlier this week that he would not enter the race, though Labour sources suggested momentum had shifted away from the self-assured Ilford North MP even before Mr Burnham arrived at Euston. Karl Turner, the longstanding Labour MP for Kingston upon Hull East who now sits as an independent, is also keen to point out the problem facing Mr Burnham’s rivals.Andy Burnham looks set to take over as Prime Minister from Sir Keir Starmer by the end of the summer | GETTYIn direct response to a post by Mr Carns, the jury reforms rebel wrote: “Al, wouldn’t much fancy a scrap with you to be fair, but if you’ve got the numbers, get yourself into the contest. If you haven’t, stop giving it large.”And herein lies the reason why Mr Burnham’s coronation could cut short Sir Keir’s stint in Downing Street. If nobody else can amass the 81 supporters needed, then Mr Burnham has the contest completely sealed up before it’s even started.Sir Keir, who looks set to leave No10 as the most unpopular Prime Minister in recent political history, will also have another damning accolade to boot. Sir Keir Starmer announced he was stepping down on Monday | PAWhile the lawyer-turned-politician occupied No10 for longer than Liz Truss and Rishi Sunak combined, Sir Keir will go down as the shortest-serving Labour Prime Minister in the party’s 126-year history. If Mr Burnham is crowned Prime Minister at the earliest date possible, Sir Keir will only spend 746 days in Downing Street. However, if a Labour MP is brave enough to challenge Mr Burnham, Sir Keir might find himself in No10 until late August.Labour’s timetable allows for a ballot of party members and affiliated supporters between August 6 and August 27.Al Carns quit from the frontbench after John Healey resigned as Defence Secretary | GETTYA vote of the party faithful might expose the extent of Labour’s decline under Sir Keir.Almost 500,000 people voted when Sir Keir stood for the Labour leadership in 2020.However, just 160,000 cast ballots to vote for Angela Rayner’s successor as deputy Labour leader last year.The final result, provided Mr Burnham goes up against another wannabe Labour leader, would be announced on August 29.