A close friend and adviser of Jeremy Corbyn has revealed how a “corridor cabal” of his top aides sabotaged Labour’s election campaign, blaming them for the party’s catastrophic defeat.

The explosive post-mortem heaps responsibility on the key allies – director of communications, Seumas Milne; chief of staff, Karie Murphy; and Andrew Murray, a veteran communist – for organisational chaos and for “suffocating” the leader himself.

It carries huge weight because it was written by left-winger Alan Simpson, a former Socialist Campaign Group MP and flatmate of Mr Corbyn, who returned to frontline politics to advise him on sustainable economics.

It also follows an internal report that was widely ridiculed for excusing Mr Corbyn and claiming his Brexit policy, backing a new referendum, played the “decisive role” in handing Boris Johnson victory.

In stark contrast, Mr Simpson accuses the leader’s closest aides of “catastrophic misjudgement and ill-focused organisation” and an “obsession” with controlling both the leader and his message.

“Jeremy will inevitably carry much of the blame,” reads Mr Simpson’s submission to an independent review into the election disaster.

“But Labour’s deeper problems lie more in the cadre of senior advisers surrounding Corbyn. None should be allowed within a million miles of Labour’s rebuilding.

“People who’d never negotiated anything more than an extended tea-break were left in charge of the policy sifting process.”

Even Mr Corbyn’s acknowledged strength – his “campaigning zeal” – was turned into a negative by his office’s “siege mentality”, it argues.

“Goodness knows how many rail miles Jeremy clocked up, but it never became the ‘leadership’ peg the public were looking for,” Mr Simpson wrote.

“Within the Loto [Leader of the Opposition’s Office] comfort zone, activity passed for strategy, when there was none.

“Instead of leading a mass movement, with a hugely empowered, devolved power base, Jeremy ended up with a corridor cabal.”

The battle to replace Jeremy Corbyn as Labour leader Show all 8 1 /8 The battle to replace Jeremy Corbyn as Labour leader The battle to replace Jeremy Corbyn as Labour leader Keir Starmer The former director of public prosecutions undoubtedly has announced that he is standing for the leadership. He is highly-regarded by both left-wingers and centrists in the party. As Labour’s shadow Brexit secretary, he played a key role in the party’s eventual backing of a second referendum. Before becoming an MP, he was a human rights lawyer - conducting cases in international courts including the European Court of Human Rights. Launching his bid, Starmer said that Labour must listen to the public on how to change "restore trust in our party as a force for good." A YouGov poll places him comfortably in the lead as the preferred candidate of 36% of party members EPA The battle to replace Jeremy Corbyn as Labour leader Lisa Nandy Wigan MP Lisa Nandy has announced she wil stand for the leadership. In a letter to the Wigan Post she said she wanted to bring Labour "home" to voters in its traditional strongholds who have abandoned the party. Nandy went on to say that she understands "that we have one chance to win back the trust of people in Wigan, Workington and Wrexham." A YouGov poll shows that Nandy is the first preference for 6% of partymembers. Getty The battle to replace Jeremy Corbyn as Labour leader Rebecca Long Bailey A key ally of the current left-wing leadership of the party, the Salford & Eccles MP is viewed in some quarters as the natural successor to Mr Corbyn and describes herself as a “proud socialist”. Highly regarded by the shadow chancellor, John McDonnell. She won also won plaudits for her performance filling in for Corbyn both at prime minister’s questions and during the general election debates. The shadow business secretary grew up by Old Trafford football ground and began her working life serving at the counter of a pawn shop. Launching her leadership bid, Long Bailey said the party needs to make the positive case for immigration as a "positive force." She also broke with Corbyn over Trident, saying "If you have a deterrent you have to be prepared to use it." PA The battle to replace Jeremy Corbyn as Labour leader Angela Rayner - Deputy leadership Shadow education secretary Angela Rayner has joined the contest for deputy leadership of the party. After ruling herself out of running for the leadership, the Ashton-under-Lynne MP launched her bid for deputy warning that Labour faces the "biggest challenge" in its history and must "win or die." She is close with leadership contender Rebecca Long Bailey PA The battle to replace Jeremy Corbyn as Labour leader Rosena Allin-Khan - Deputy leadership Shadow sport minister Rosena Allin-Khan said Labour need to listen with "humility" to lost voters as she launched her bid for the deputy leadership. Writing in The Independent, the MP for Tooting refelcted: "We shouldn’t have ignored the warning signs in Scotland, and now we’ve paid the price in northern England, across the midlands and in Wales." PA The battle to replace Jeremy Corbyn as Labour leader Dawn Butler - Deputy leadership Shadow women and equalities secretary Dawn Butler was first to announce her bid for the deputy leadership. The Brent Central MP has served in Jeremy Corbyn's shadow cabinet since 2016 PA The battle to replace Jeremy Corbyn as Labour leader Ian Murray - Deputy leadership Labour's only MP in Scotland said that the architects of the party's "catastrophic failure" in the December election can not be allowed to lead the party forward PA The battle to replace Jeremy Corbyn as Labour leader Richard Burgon - Deputy leadership Shadow justice secretary Richard Burgon is standing as a continuity candidate, flaunting his loyalty to Jeremy Corbyn and saying it is wrong to blame the current leader for the election defeat PA

Speaking to The Independent, Mr Simpson said, of the team led by Mr Milne, Ms Murphy and Mr Murray: “This faction took control of everything that came through Jeremy’s office – they ended up not protecting him, but suffocating him.”

The former MP for Nottingham South has impeccable pro-Corbyn credentials as an opponent of the wars in Iraq and Afghanistan and, in the 1990s, of Tony Blair’s abolition of Clause IV.

His verdict, which he sent to the review being carried out by Ed Miliband and others in the Labour Together group, also castigates the leader’s team for such failures as:

Pursuing the election in December instead of leaving the prime minister to “wallow in the Brexit mess” until the spring or summer of 2020. “The trouble is that many of those closest to Corbyn always looked as if they wanted Brexit anyway,” Mr Simpson wrote;

Failing to bring in well-known outside figures with fresh ideas – as David Cameron successfully did – to boost public support for policy shifts;

Blocking an eye-catching proposal to shift funding from private to public transport – “Germany’s 10 per cent cut in rail fares shows how popular such radical changes can be”;

Blocking radical proposals on the environment and on ‘greening’ energy through decentralised distribution.

“The most repeated shadow-ministerial complaint was about delays in getting radical policy proposals through the Loto soup,” Mr Simpson said.

A manifesto pledge to nationalise the Big Six energy firms – rather than set up rival local networks to achieve a gradual shift – was inserted at the last minute without even the agreement of John McDonnell, the shadow chancellor, The Independent also understands.

Since the election debacle, there has been growing anger that Mr Milne and Ms Murphy have retained their positions even as Labour has shed many other staff and is drawing up proposals for other departures.

Mr Corbyn also faces a backlash for nominating Ms Murphy for a peerage, which is expected to be confirmed in an announcement this month.

She has faced allegations of bullying and questions over her role in the handling of Labour’s antisemitism scandal, which is still under investigation by the Equality and Human Rights Commission (EHRC).