In an joint statement they expressed 'serious concerns' over the disciplinary action taken against the former leader in the wake of a damning anti-Semitism report.
As Sir Keir toured broadcast studios on Thursday offering a full-throated apology to mark a clean break with the past, Mr Corbyn caused uproar when he said the scale of anti-Semitism had been 'dramatically overstated for political reasons'.
His swift stripping of the party whip has flared up old battle lines as figures on the left such as John McDonnell and Diane Abbott rode to Mr Corbyn's support.
Powerful union bosses who bankroll the party, including Unite's Len McCluskey, have now waded in to brand the suspension 'ill-advised and unjust'.
Labour's reignited factional war was stepped up today after seven left-wing unions took aim at Sir Keir Starmer (left) for suspending Jeremy Corbyn (right)
Powerful union bosses that bankroll the party, including Unite's Len McCluskey (pictured), have now waded in to brand the suspension 'ill-advised and unjust'
Their joint statement to Sir Keir, seen by the LabourList website, says: 'The publication on Thursday of the EHRC report ought to have marked a moment of reflection and repair for our party. Instead, an ill-advised and unjust suspension has caused division.
'We therefore call upon the leader, Keir Starmer, the general secretary David Evans and the NEC to work now with us as affiliated unions to repair this damage.'
The union barons added they could 'not comprehend' why Sir Keir would deliberately sabotage the opportunity 'to unite the party'.
It ends: 'We therefore urge Keir to work with us on a fairer, unifying way forward.'
It was signed by the bosses of Unite, the Bakers Union, ASLEF, CWU, FBU, NUM and the TSSA.
Sir Keir claims to have adopted a zero tolerance approach to anti-Semitism after the scourge was allowed to breed in Labour's ranks during the previous leadership.
But Corbyn loyalists yesterday accused him of tearing up the rulebook after the Labour leader struggled to say the exact breach the left-winger had committed.
Mr Corbyn caused uproar when he said the scale of anti-Semitism had been 'dramatically overstated for political reasons'
The charges against Labour in damning 130-page report
During Mr Corbyn's tenure he commanded the overwhelming support of the grassroots but had to face down numerous backbench revolts.
Sir Keir's pitch for leader vowed to stamp out the factionalism that had dogged the party.
Shadow business minister Lucy Powell today said she did not think the Labour Party was 'split' in the aftermath of a damning anti-Semitism report.
On the former leader suspension from the party, she told BBC Breakfast: 'I hope that these things internally within the Labour Party will resolve themselves in due course and I am sure they will.
'There is now a due process taking place. Frankly as a party we have got the T-shirt on disunity. We have seen how that plays out.
'I am sure that none of us want to go back to that era for the Labour Party.
'I am sure we will move on as one united party at what is a very very critical time for our country and for people's lives and livelihoods.'
An investigation by the Equality and Human Rights Commission found Labour was responsible for unlawful acts of harassment and discrimination.
The EHRC investigation found evidence of 'political interference' by then leader Mr Corbyn's office in the complaints process.
Interim chairwoman Caroline Waters said there had been 'inexcusable' failures which 'appeared to be a result of a lack of willingness to tackle anti-Semitism rather than an inability to do so'.
The watchdog identified three breaches of the Equality Act relating to political interference in complaints, failure to provide adequate training to those handling anti-Semitism cases, and harassment.
The party has been served with an unlawful act notice and has been given until December 10 to draft an action plan to implement the report's recommendations. The notice is legally enforceable by the courts if not fulfilled.
He told the virtual 'Stand with Corbyn' rally: 'Numerically we know that the incidences of anti-Semitism may have been small but these things aren't measured by numbers, but by the seriousness of the issue, by the effect that they can have.
'The evidence shows that some people, yes, may have overestimated the numbers of party members involved - but that isn't the issue.
'The stain of anti-Semitism is not measured by numbers, but by the appalling offensive nature and existence. And as Jeremy said time and time again, one anti-Semite in our party is too many.'