The episode has triggered widespread anger among supporters of rival campaigns, who have accused the leader’s team of manipulating the race to rescue their favoured candidate.
Ms Long-Bailey was investigated after breaching a ban on promoting campaigns to members using party data – an offence some Labour MPs viewed as potentially serious enough for her to be thrown out of the contest altogether.
Instead, party officials – including leading figures in the Unite union, which backs the shadow business secretary – decided it was “reasonable” for candidates to contact their own local members.
“It’s clear the Labour Party is making it up as it goes along and deliberately engineering the rules in favour of a certain candidate,” alleged Wes Streeting, a supporter of Jess Phillips until she dropped out.
A second MP said: “This shows how they manipulate the system, they are doing everything they possibly can to help their candidate. They are in favour of party democracy, as long they control the rules.”
The controversy follows Labour laying down a strict rule barring any MP, or their supporters, from using party membership lists to advertise their campaign to succeed Mr Corbyn.
However, when Ms Long-Bailey – who is backed by the outgoing leader’s team as the “continuity Corbyn” choice – entered the race, she emailed all members in her Salford constituency party (CLP) to ask for their backing.
“I hope you can support me in the leadership contest and please feel free to get in touch with me or the office if you want to pop in for a chat,” the shadow business secretary wrote.
Three days later, on 10 January, the procedures committee running the contest emailed all the candidates acknowledging there had been a breach of the rulebook.
However, the message – seen by The Independent – concluded: “After due consideration, the procedures committee has agreed it is reasonable for candidates for leader or deputy leader to contact members in their own CLPs to inform them of their intentions.”
Labour refuses to divulge the membership of the all-important committee, but The Independent understands it includes leading figures from Unite, including party treasurer Diana Holland.
The email announcing the rule change was sent by Jennie Formby, the party’s general secretary, another big supporter of Mr Corbyn and his project.
The original bar on campaigning using membership lists did not set out the punishment for doing so, stating: “Any breach of this may result in a sanction determined by the procedure committee.”
However, other campaign teams believed a breach of any rule could trigger financial curbs on spending, exclusion from certain events – or even expulsion in certain circumstances.
The row comes on the back of existing anger that Ms Long-Bailey is already able to use the 40,000-strong membership list of the left-wing group Momentum, which is supporting her.
Mr Streeting said the retrospective rule change “goes against the basic sense of fair play that Labour members expect”.
“Rebecca Long-Bailey already has an organisational advantage because Momentum has huge amounts of data from two previous leadership elections,” he added.
“If you want to be the leader of the Labour Party – and believe you can win a general election – you should be able to fight by the same rules with fair access to members, as a fundamental principle.”
The campaign team for Ms Long-Bailey was asked to comment on the criticism, but declined to do so.
A Labour Party spokesperson said: “This is nonsense. All candidates are being treated equally and fairly, and to suggest otherwise is wrong and offensive.”
Sir Keir, the shadow Brexit secretary, is the favourite to win the leadership race when the result is declared on 4 April.
However, Ms Long-Bailey – despite a difficult start to her campaign – remains a strong contender with the backing of Momentum and Unite.