The Electoral Commission has said it will not reopen an investigation into Labour Together, the think tank previously led by the prime minister’s chief of staff Morgan McSweeney.

In 2021, the commission investigated and fined Labour Together for late and inaccurate reporting of its donations.

Earlier this week the Conservatives urged the commission to open a fresh inquiry arguing that “new information” had raised more questions about the donations.

However, in a statement on Friday, the commission said it had “thoroughly reviewed this information and found no evidence of any other potential offences”.

Conservative chairman Kevin Hollinrake said the decision was “wrong” and reiterated his call for the commission to publish its full report into the donations.

A Labour Party spokesperson said: “In a pathetic and desperate attempt to stay relevant, the Tories only hope is to throw mud at the wall and hope something sticks. There isn’t a low that they won’t stoop to.”

Mr McSweeney was director of Labour Together from 2017 to 2020, before leaving to manage Sir Keir’s Labour leadership campaign and later becoming his chief of staff in Downing Street.

In 2021, the Electoral Commission fined Labour Together £14,250 for failing to deliver donation reports within 30 days, inaccurately reporting donations and failing to appoint a responsible person within 30 days of accepting a donation.

During Mr McSweeney’s time at Labour Together, the group received donations totalling more than £700,000 that were not declared.

Following its investigation, the commission only published a summary of its findings and in 2023 it refused a request made under the Freedom of Information Act to release a copy of its full investigation.

On Monday, Hollinrake asked the commission to revisit the case pointing to leaked emails – apparently from a Labour Together lawyer to Mr McSweeney – that had come to light.

The Conservative chairman suggested the correspondence could be seen as evidence the Electoral Commission had been “knowingly misled”.

He said the implication of leaked emails was that Labour Together “chose not to report those donations” to stop the then Jeremy Corbyn-led Labour Party knowing “who was bankrolling their secretive political campaigning, and to keep their work below the political radar”.

Mr McSweeney did not reply to a request for a response to the allegations.

Labour Together said the group had “proactively raised concerns about its own reporting of donations to the Electoral Commission in 2020”.

“The Electoral Commission’s investigation, with which Labour Together fully co-operated, was completed in 2021. The outcome was made public and widely covered by the media at the time,” a spokesperson said.

“Since this time, we have taken measures to ensure Labour Together is fully compliant with all Electoral Commission regulations.”

On Friday, the commission responded to Hollinrake’s letter saying: “We have thoroughly reviewed this information and found no evidence of any other potential offences.

“We are confident that the initial determination and sanction were appropriate. We are therefore not reopening the investigation.”

Hollinrake said: “This is not over, we will continue to reveal more evidence, and continue to push for a full investigation by the parliamentary standards commissioner into Keir Starmer.

“The British public deserve the full truth, not another cover-up, and the Conservatives will continue to fight until they get that.”

Hollinrake has called for an investigation into whether Sir Keir failed to declare support – such as polling and speechwriting help – from Labour Together during his 2020 leadership bid.

A Labour source previously told the BBC: “Neither Keir, nor his leadership campaign accepted monetary or in kind donations from Labour Together during the leadership election.”