Keir Starmer’s most senior advisers were briefed about an “indefensible” investigation into journalists writing critical pieces about the Labour Together thinktank, according to a newly released document.

Among the aides who received updates on the probe, commissioned by the thinktank’s director, Josh Simons, were Morgan McSweeney, the former chief of staff to the prime minister.

McSweeney declined to comment on the revelation, but the disclosure has raised questions about Starmer’s own knowledge about the affair, which is the subject of a review by the Cabinet Office’s propriety and ethics team.

Josh Simons, later a minister, was Labour Together’s director when he commissioned an investigation in 2023 into the source of stories about £740,000 undeclared donations made to the thinktank when it was previously run by McSweeney. The thinktank was fined £14,250 in September 2021.

Six months before Labour won the 2024 general election, Simons wrote to McSweeney and Paul Ovenden, who was Starmer’s director of political strategy, to update them on investigation run by Apco Worldwide, a US-owned PR firm.

Tom Harper, Apco’s senior director, had been investigating the background and motivations of the journalists behind articles and social media posts, including Henry Dyer from the Guardian.

“Tom Harper (on copy) will be delivering his report on Paul Holden on Monday,” Simons wrote in an email on 14 January 2024. “Could we find a time for Tom, Morgan, Paul and me to zoom or meet in HQ sometime after that next week?”

Internal reports prepared by Apco – who were paid more than £30,000 by Labour Together – named the Sunday Times journalists Gabriel Pogrund and Harry Yorke, as well as Dyer, Declassified UK’s John McEvoy and journalists from other outlets as “significant persons of interest”.

The final report delivered by Harper included claims about the faith and relationships of Pogrund, who co-authored a Sunday Times investigation. It suggested Pogrund’s reporting could be seen as “destabilising to the UK and also in the interests of Russia’s strategic foreign policy objectives”.

Simons, who was made a minister after the general election, was in February forced to step down when details of the investigation emerged. He has previously stated he was “surprised and shocked” by Apco’s report, claiming it had gone beyond the terms of the contract.

The email sent to McSweeney and Ovenden was discovered after a subject access request by Paul Holden, a journalist whose research formed the basis of the Sunday Times investigation, and who was also a subject of interest in Apco’s probe.

Simons has said he commissioned the investigation as he suspected there had been a hack of the Electoral Commission of sensitive Labour Together materials. Apco’s report suggested Holden was “part of a far-left network …  which disseminates pro-Russian propaganda”, and the coverage was part of a “coordinated effort to discredit Starmer”.

McSweeney and Ovenden have since resigned from Downing Street over separate issues.

It is understood McSweeney knew about the commissioning of the Apco report and approved of it but the material presented in the meeting was heavily redacted so that the journalists names were unidentifiable.

It has been denied that Starmer was ever aware of the investigation or its findings.

Holden has called for a full parliamentary inquiry into Labour Together. “These documents now show that the deeply invasive investigation into me, my family, colleagues and associates was, in effect, a joint project on the part of Labour Together, Apco and the highest levels of the Labour party,” he said. “This was an appalling attack on public interest investigative journalism.” Labour said it supported press freedom:

Labour Together recently changed its name to ThinkLabour. Alison Phillips, its chief executive, said the organisation had changed under her leadership. “I remain shocked at the work undertaken by Apco in 2023 for Labour Together. It was indefensible,” she said.

Simons stood down as an MP this week in order to trigger a byelection in which Andy Burnham hopes to return to Westminster, in an effort to replace Starmer. Harper left Apco after it emerged he had told a contractor to “get rid of” documents related to the Labour Together work.