A senior executive at TikTok helped write the Labour manifesto while still working at the Chinese-owned technology company, he has said.

James Lyons, who left his post as Keir Starmer’s director of strategic communications last week, wrote on LinkedIn that he was asked last year to help write the party’s election pledges while he was still working for the social media platform.

His work on the manifesto renews questions about the close relationship between Labour and major technology companies, who have enjoyed regular access to senior ministers in Starmer’s government.

Lyons wrote his LinkedIn post mainly about his time in government, where he was director of strategic communications for 11 months. As part of the post, he talked publicly for the first time about how he originally became involved in Starmer’s project.

“The ask to enter No 10 came out of the blue,” he said. “I had helped Labour during the election by rewriting the manifesto to make it more compelling (and remove bear traps where I could).

“To ensure security this was done, in spare time over evenings and weekends, on a laptop that didn’t leave the party HQ. Rightly, there was never any promise of a job.”

A Labour spokesperson said: “Thousands of people volunteered their spare time during the general election to help Labour end 14 years of Tory chaos. All of this was done in their own time.”

Lyons, a former journalist at the Mirror and the Sunday Times, first joined TikTok as the company’s head of European corporate and policy communications.

While there he earned a reputation as a fierce defender of the company, especially in the face of allegations that its Chinese ownership could undermine the data security of its western users.

He was still in that role in the run-up to the election, when he went to work for Labour on the manifesto. Those working for Labour at the time say he was drafted in not to formulate policy but to rewrite the text of the manifesto to make it more compelling.

Labour’s pre-election manifesto talked optimistically about the benefits the technology industry could bring to the economy, though relatively little about social media policy.

The one policy which could have had an impact on TikTok’s business was a promise to “build on the Online Safety Act”, adding that the party would “explore further measures to keep everyone safe online, particularly when using social media”.

TikTok itself remains banned on government devices over concerns that user information could be accessed by the Chinese government, though the government does now have an official account on the platform.

Lyons said in his post: “Social media now rivals broadcasters and newspaper readership has fallen by around 75% since I became a lobby hack [journalist] 25 years ago.

“Lots of Whitehall comms haven’t changed much in eight years since I left SW1 – and some have barely changed since I arrived in 2000. At No 10, with the New Media Unit and a renewed focus on consumer outlets, we were going where the voters are.”

There is no suggestion he broke any rules.

Since entering government, Labour has faced frequent criticism both for its attempts to forge a rapprochement with Beijing and for the frequency with which its senior ministers have met large technology companies.

The Guardian revealed in May that Peter Kyle had met large technology companies far more frequently when he was technology secretary than his predecessor, Michelle Donelan, had.

The regular contact – which included 28 meetings in a six-month period – came just as Starmer was advocating cutting-edge technology and artificial intelligence as the solution to many of the country’s problems with public service provision.

The prime minister repeated that message on Tuesday at the first meeting of his newly appointed cabinet.

According to a Downing Street readout of the meeting, Starmer told ministers “it was incumbent on the government to go further and faster in reducing the size of the civil service, adopting technology and AI across public services”.

A Conservative source said: “The Labour government has been cosying up with China since day one, the threat it poses to our national security be damned.

“Now it emerges that a senior figure working for a Chinese firm was a central author of Labour’s manifesto. Labour … have serious questions to answer.”