Rebecca Long-Bailey has leapfrogged Sir Keir Starmer in the race to be the next Labour leader, a new poll has suggested.
An earlier survey had put Sir Keir narrowly ahead.
The new poll by Survation indicated that Ms Long-Bailey, the shadow business secretary, would secure 42% of first preference votes to the shadow Brexit secretary’s 37%.
According to the results, Jess Phillips would get 9% of first preferences, Lisa Nandy 7% and shadow foreign secretary Emily Thornberry just 1%.
However, more than a third of those questioned said they had not decided who they would vote for in the leadership contest.
Only 22% said they were certain they would not change their mind in the remaining 11 weeks of the race.
The poll asked readers of Labour List, a website focused on party news, for their preferences and then weighted the results to reflect the membership.
It suggests that the five-candidate contest is wide open, with Ms Long-Bailey, who is backed by senior figures on the left of the party, including shadow chancellor John McDonnell, looking most popular with signed-up members.
The Salford and Eccles MP had come a long way second to Sir Keir in the round of nominations by MPs and MEPs, picking up 33 supporters to the Brexit spokesman’s 88.
But the new Labour leader will be decided by party members, who currently number around 500,000, using a preferential voting system, with members able to order the candidates in a list from one to five.
In order to progress to the postal ballot of members and supporters, candidates for both leader and deputy must get the support of 5% of local parties or three affiliates, including two unions, by 14 February.
If no candidate wins 50% of the vote during the first round of counting, the last place contender is eliminated and their second preference votes are re-distributed.
This process goes on until one of the candidates secures a majority.
The winner will be announced at the start of April
Candidates are due to face off during party-organised leadership hustings held across the UK, with the first scheduled for Saturday in Liverpool.
The Survation poll made Angela Rayner the overwhelming favourite to win the deputy leadership contest.
It suggests the shadow education secretary would win the battle on first preferences alone, taking 60% of the vote.