Keir Starmer is facing calls by MPs for an inquiry into the commissioning of a report that made “baseless claims” about journalists who were investigating a thinktank linked to the prime minister.

The calls add to pressure on the Cabinet Office minister Josh Simons, who commissioned a report in 2023 on journalists investigating Labour Together, the thinktank that would help propel Starmer to power.

The research was paid for and subsequently reviewed by Simons when he was director of Labour Together, according to sources and documents seen by the Guardian.

In an agreement addressed to Simons, drawn up by Apco Worldwide, the PR consultancy agreed to “investigate the sourcing, funding and origins” of a November 2023 Sunday Times report about the thinktank, in addition to other journalistic investigations into the group.

The Sunday Times reported that the contents of Apco’s investigation were informally shared with Labour figures in 2024, including present cabinet ministers and special advisers. The report contained allegations about Gabriel Pogrund and Harry Yorke, journalists at the paper, which then spread around Westminster.

The paper reported that Tom Harper, Apco’s senior director and a former Sunday Times employee, wrote that he had examined the “sourcing, funding and origins of the Sunday Times story” using documents and “discreet human source enquiries”.

Harper was said to have made “baseless claims” that the emails underpinning the published story were likely to have come from a suspected Kremlin hack of the Electoral Commission.

“The likeliest culprit is the Russian state, or proxies of the Russian state,” he reportedly wrote. Apco’s report was also said to have referred to Pogrund’s Jewish background and made baseless allegations about his faith, upbringing and personal and professional relationships.

Simons was close to the prime minister’s recently departed chief of staff, Morgan McSweeney, who previously ran Labour Together and whose own role in the operation to gather material on journalists is under scrutiny.

An investigation into Apco’s research has been launched by the Public Relations and Communications Association’s standards committee, and this has been welcomed by Simons.

He said in a statement: “I was surprised and shocked to read the report extended beyond the contract by including unnecessary information on Gabriel Pogrund.”

Simons said he had asked for the information to be removed before passing the report to the intelligence agency GCHQ. No other British journalists were investigated in any document he or Labour Together ever received, Simons said.

John McDonnell, the veteran Labour backbencher and former chancellor, said on Saturday he had written three times in recent weeks to the general secretary of the Labour party to call for an independent inquiry into the affair.

“I copied Keir Starmer into each request,” McDonnell said. “Clear to me as secretary of the NUJ’s parliamentary group if true this is unacceptable.”

Another Labour MP, Karl Turner, said the prime minister needed to look into the affair himself and should meet McDonnell to discuss it.

The contract showed that Simons asked for information specifically on the sources for a book by Paul Holden about McSweeney’s role in Starmer’s rise, as well as related articles by the US journalist Matt Taibbi.