Angela Rayner suffered a major blow to her expected run at the Labour leadership after she failed to even hold her local council.
Labour lost its majority on Tameside council, the authority which includes the former deputy prime minister's Greater Manchester constituency.
The party lost control of the council after an uninterrupted run of 47 years as Reform took all but one of the seats up for election.
Sir Keir Starmer's party lost 16 councillors while Reform UK gained 18 while the Conservatives lost two and the number of independents stayed the same.
It is likely to raise questions about whether Ms Rayner is popular enough to turn around Labour's fortunes in the event there is a leadership contest.
A national drubbing is likely to reignite speculation about Sir Keir Starmer's leadership of the party and the country.
Former cabinet minister Ms Rayner is expected to throw her hat in the ring in the event that the Prime Minister decides to step down.
She is said to have lined up the necessary 81 MPs required to launch a challenge against the PM but is not expected to make the first move.
She remains under investigation by the HMRC for her tax affairs over her failure to pay £40,000 she owed in stamp duty.
The Health Secretary Wes Streeting is also said to have amassed the support of 81 MPs and will stand in any contest, though not trigger one.
Meanwhile, Greater Manchester mayor Andy Burnham's path back to Westminster will be complicated by the surge in support for Reform in the North West.
He would need to win a by-election in order to stand for the leadership but safe seats in the area look in doubt after gains by Nigel Farage's party in the locals.
On a disastrous night, Labour lost control of several other authorities including Wandsworth, Westminster and Hartlepool.
In Tameside, Labour lost 14 of the 15 seats it was defending - enough to deprive it of a majority and leave the council with no group large enough to have overall control.
The council is home to Ashton-under-Lyne, the constituency she has represented since 2015, and Labour have held it since 1935.
All eight of the council seats within the constituency's boundaries were controlled by Labour until today -- when Reform won all but one of them.
The new state of the parties is Labour 25 seats, Reform 19, independents eight and the Conservatives five.
The council also covers the constituency of Jonathan Reynolds, the chief whip, and Hannah Spencer, the Green MP who recently won the Gorton and Denton by-election.
Interim Chair of Reform UK in Tameside, Rob Barrowcliffe, said 'no branch in the country has knocked on more doors' than in Tameside and Gorton, which had been under majority Labour control since 1979.
'We are normal, hard-working, competent, decent people that have simply had enough,' he told BBC Radio Manchester.
He added: 'Next year if we replicate this result we can take overall control.'
'The council is now in no overall control, which is a precarious situation for the leading group.'
'Our guys have zero political experience and people see that as a negative thing. I see it as a complete positive.'
She has represented the seat since 2015, and Labour have held it since 1935. All eight of the council seats within the constituency's boundaries were controlled by Labour until today -- when Reform won all but one of them.
It was also reported that Ms Rayner had suffered a further blow to her ambitions after she failed to win over a group of 100 Labour MPs.
She secretly met former transport secretary Louise Haigh, the leader of the Tribune group of MPs, to try to win over the caucus.
But the i paper reported that she left empty handed, with the group expecting to line up behind Mr Burnham in the event of a contest.
Ms Haigh is said to have always been 'prickly' towards Ms Rayner for continuing with her leadership ambitions despite her tax issues.
It comes after the former transport secretary was forced to quit over a fraud offence connected with a stolen mobile phone.