Editor of the Sunday Times, Ben Taylor, said the “excuses” of those at the top of Labour Together “won’t wash”.

“When the report came in, Labour Together people, some of whom are now in the cabinet, were quite happy to talk about its conclusions widely around Westminster – claims that the Sunday Times was being used by the Russian state to run disobliging stories about Labour and frankly cast doubt on the title and their reporters,” he told BBC Radio 4’s Today programme.

Speaking before the investigation had been announced, Labour MP John McDonnell said he had written to Labour four times about the subject after being approach by Democracy for Sale, who initially reported the story., external

McDonnell told the Today programme he thought these were “serious allegations” and that the “Labour Party should get on top of it”.

He said his reply from the party’s general secretary said Labour Together was not a Labour organisation and referred him to the complaints department about individual members of the party.

“I said that doesn’t meet the seriousness of this case,” he added.

Following the announcement, McDonnell posted on X: “So Cabinet Office is ‘assembling the facts’ into the activities of a Cabinet Office minister… could I suggest this hardly inspires confidences this will be the thorough and independent inquiry into the activities that is needed.”

Another Labour MP, Richard Burgon, said: “The Labour Party needs to start taking these allegations very seriously. That means an independent investigation.”

Alison Phillips, the chief executive of Labour Together who joined after the APCO report was commissioned and delivered, said: “I am committed to ensuring Labour Together aspires to the highest standards of probity at all times.

“We stand ready to support the PRCA [and other relevant governance bodies] with its review of this issue.”

Separately, in an all-staff email seen by the BBC, Phillips said: “The allegations of recent days have been shocking to read and I know they have been deeply concerning for many of you.

“As a former journalist and editor, it should come as no surprise that I was horrified that investigators hired by Labour Together would look into the background and sources of reporters even if I am assured that this was not the intention.”