A senior Labour politician has written to civil servants demanding a formal investigation into a Conservative cabinet minister accused of breaching the ministerial code.

Welsh Secretary David TC Davies posted a short film on X, formerly Twitter, last week, attacking Labour’s plans to expand the size of the Welsh Parliament.

But Labour opponents say Mr Davies was recording in his government office, in contravention of the ministerial code, which says they “should not generally be used for party or constituency activities”.

Jo Stevens, Labour’s shadow Welsh secretary, has now written to the director of the Welsh Office – the civil servant responsible for his office – to ask them to establish whether a breach has occurred and to investigate how it happened.

In her letter, she said: “The Welsh Secretary posted on the social media site X a video filmed in his private office, which contained explicitly party-political content, criticising the ‘Welsh Labour Government’ for its proposals on Senedd reform, and endorsing the opposition to those proposals from the “Welsh Conservative Group”.

“He closed with the line: “I know which side I’m on, I wonder what Welsh Labour are going to do today.”

She added: “In your capacity as the director of the Secretary of State’s Office, can I ask you to establish whether a breach of the Ministerial Code took place in this instance, and if so, what action will be taken to investigate how that happened, to ensure that it never happens again, and to issue a formal apology from the Secretary of State.”

Mr Davies has been contacted for comment.

A source close to the minister previously told the Guardian newspaper they had been unable to film outside his office “for security reasons”.

In 2023, MP Lee Anderson was rebuked for using a parliamentary rooftop to film a promotion video for his £100,000-a-year GB News show.

The Serjeant at Arms, who is responsible for upholding order in the Commons, contacted Mr Anderson, who was the Conservative Party deputy chairman at the time but now sits as Reform UK MP, over his Twitter clip to remind him of the rules.