Labour must end the factionalism which developed under Jeremy Corbyn, Sir Keir Starmer said after being “shocked” by the contents of a leaked dossier which detailed splits in the party’s headquarters.

Sir Keir said the party must unite and concentrate on getting Labour into a position to form the next government.

The leak of the document reignited Labour’s bitter civil war between left-wing supporters of Mr Corbyn and the party’s centrists.

We absolutely have to turn our back on factionalism and that was the whole basis of my leadership campaign

The 860-page leaked document found “no evidence” of anti-Semitism complaints being handled differently from other forms of complaint, or of current or former staff being “motivated by anti-Semitic intent”.

But it found that “factional opposition” in the upper echelons of the party towards Mr Corbyn contributed to “a litany of mistakes” which hindered the effective handling of the issue.

Sir Keir told BBC Radio 4’s Today programme: “This is a leaked report into what was apparently happening before I became leader. I was shocked by what I saw and the circumstance in which it all came about.

“That’s why I have ordered an inquiry, which I want to be professionally done, independently done and quickly done, because our party needs to unite and face the future and get back to our historic purpose, which is getting a Labour government in so that we can actually change lives for millions of people for the better.”

He added: “We absolutely have to turn our back on factionalism and that was the whole basis of my leadership campaign.”

Allies of Mr Corbyn claimed the document showed that elements within the party hierarchy undermined efforts to win the 2017 general election.

The Socialist Campaign Group of Labour MPs – including former shadow cabinet minister Diane Abbott, John McDonnell and Richard Burgon – has called for the document to be officially published with an emergency meeting of Labour’s ruling National Executive Committee to consider its findings.

They said “it contains revelations of senior officials undermining the 2017 general election campaign and suggests there are cases to answer on bullying, harassment, sexism and racism”.

Mr Corbyn’s constituency party’s Twitter account said the NEC should “recommend urgent changes of party structure and staffing practices to guarantee our HQ campaigns on Labour’s socialist policies”.

Unite leader Len McCluskey said the whole labour movement will be “shocked” by the revelations contained in the leaked report.

Writing for the LabourList website, he said: “Even those of us well aware of the dislike many officials held for Jeremy Corbyn’s leadership did not imagine that their bile would go so far as actively wishing for – even working for – Labour’s defeat in elections, including the 2017 general election.

“Every activist will remember how agonisingly close we came to defeating Theresa May then. I have no doubt that had the party leadership enjoyed the full-on support of all Labour MPs and of Party HQ, both of which it had the right to expect, the result would have been closer still.

“Let us be clear what the officials whose cynical, abusive and factional conduct has now been exposed were actually doing.

“In working for a Labour defeat, they were working for a Tory victory – that is to say, empowering the party that stood for austerity and a ‘hard Brexit’.

“These politically-crooked officials were prepared to risk dramatic damage to the interests of the British economy and working people just in order to scratch their factional itch.

“And some of these officials have now secured peerages or been dubbed this-or-that of the British Empire ‘for services to the Labour Party’. If they are to keep these distinctions, at the very least the citations should be changed to ‘services to the Tory Party’.

“I know there are tens of thousands of Labour Party members, many of them also in my union, whose dismay at these revelations may lead them to wonder why they should stay in a party where such things can happen. Let me urge them to remain with the party and get behind our newly-elected leadership as they handle this crisis.”

The GMB union branch representing Labour Party staff welcomed the inquiry ordered by the new leadership into the report, saying in a statement: “We are deeply concerned with the report, and the means by which it was commissioned, created, and made public.

“It is also disappointing that much of the report diverts from the scope of its declared intention, which was to look into the Labour Party’s response to anti-Semitism.

“It is also extremely worrying that the report contains transcripts of private conversations between staff on private accounts, using personal equipment.

“We agree some of the selected comments attributed to senior staff are absolutely indefensible, it is worth noting that they are not being seen within context and some comments attributed are being flatly denied.

“It is also unacceptable that staff members across all levels have had their private communication accessed and their names leaked publicly.

“The Labour Party would rightly not accept this level of surveillance and intrusion into private conversations of staff in any other workplace, and we believe it therefore should never countenance such practices in the party – let alone authorise them.

“This is causing immense stress for those workers, as well as colleagues who are not named but now feel a deep sense of mistrust toward their employer.”