Unite the Union leader Len McCluskey pointed out one single label connected to Labour Party deputy-leadership candidate Rosena Allin-Khan will ensure party supporters do not back her bid. Mr McCluskey suggested the Labour MP's stance on Brexit will play a key role in whether she succeeds Tom Watson in the top position within the Labour Party. Speaking to Peston, the union leader said: "Rosena is a Remainer.
"She was one of the individuals desperately trying to make Labour a Remain party.
"The fact is, those of us who speak and have members that are rooted in our heartlands in the north and the Midlands have known for a long long time that we would suffer the consequence."
Mr McCluskey added: "The Labour leader has to listen and learn."
Ms Allin-Khan has been a vocal opponent of Brexit, repeatedly voting in support for further British integration within the European Union as well as to have the UK remain in the bloc.
McCluskey said Allin-Khan will not secure the backing of Labour voters because of her Brexit stance
She received the support of several Constituency Labour Parties including the CLP in her own constituency of Tooting, who also backed Wigan MP Lisa Nandy as Labour leader.
Mr McCluskey's union however backed the bid of shadow Business Secretary Rebecca Long-Bailey and Richard Burgon as her deputy-leader.
The leadership candidates have until February 14 to get the nominations from Constituency Labour Parties and affiliates (including trade unions) to secure a spot on the final ballot box that will decide the successor of Jeremy Corbyn.
They must get the backing of five percent of constituency parties or three affiliated bodies, two of which must be trade unions, to stand in the leadership election.
Three of the candidates – Sir Keir Starmer, Rebecca Long-Bailey and Lisa Nandy – are believed to have received the necessary support to feature on the ballot box but shadow Foreign Secretary Emily Thornberry looks set to crash out of the contest on Friday.
After the deadline, members of the party, affiliated unions and socialist societies like the Fabians will be able to cast their votes until April 2.
Labour Party members will pick their candidate in a one-member, one-vote system and the winner will be announced at a special conference on April 4.
Current Labour leader Jeremy Corbyn was forced to announce his resignation after the Labour Party took a drubbing at the last election.
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Labour lost 60 seats across the UK, including some historic seats in traditionally Labour areas that turned to the Conservative Party for the first time in decades.
A report into Labour’s humiliating election defeat, the worst since 1935, by Corbynista MPs Andrew Gwynne and Ian Lavery, exonerated the leader last month.
But Lord Ashcroft’s report lays bare the feelings over voters about Mr Corbyn’s leadership.
They raised concerns about his record on defence and national security, his links to Hezbollah and the IRA and lack of patriotism and his scruffy appearance.