The 13 unions involved are Unite, USDAW, BFAWU, UCU, RMT, ASLEF, FDA, NEU, PCS, POA, NUJ, BALPA, and the GMB.

Today’s pre-action judicial review claim letter to Mr Kwarteng explains that the new regulations are a violation of Article 11 of the European Convention on Human Rights. It also explains that the new regulations violate the EU-UK Trade and Cooperation Agreement, which commits the UK to respecting, promoting and implementing internationally recognised core labour standards, including those relating to freedom of association and the recognition of the right to collective bargaining.

The letter also argues that the business secretary has failed to discharge the obligation to consult as required by the ‘Employment Agencies Act 1973’.

Commenting in June, Mr Kwarteng said that the new amendment regulations “will give businesses freedom to access fully skilled staff at speed”, but critics argued that the new regulations would be impractical – especially when trying to replace skilled workers at quick notice.

Mr Kwarteng will have 14 days to respond before a judicial claim is filed.

Richard Arthur, head of trade union law at Thompsons Solicitors, said: “The Conservatives won the 2019 General Election on the promise of raising standards in workers’ rights, and ‘levelling-up’.

“They have been determined to do the exact opposite. Whether it’s promises to remove retained EU workers’ rights or making it more difficult to organise industrial action, this government has shown its commitment to removing the means by which workers will get the pay rises they need to see them through the costs of living crisis.

“The new agency worker regulations were supposed to appease the right of the Conservative party. That tactic didn’t work. Now we see Liz Truss talking of increasing minimum voter thresholds for industrial action, doubling the minimum strike action notice period to four weeks, and introducing a ‘cooling off’ period’ for strike action.

“Internationally protected trade union rights are being used as red meat to feed, first to Boris Johnson’s detractors, and now to the Conservative Party’s members in the leadership election.

“If the business secretary won’t see sense, then he’ll face a judicial review.”