Unions have won a “significant” victory after challenging law changes they said let agencies supply employers with workers to fill in for striking staff, a lawyer says.

More than 10 unions, including Aslef, Unite and Usdaw, took High Court action against the Conservative Government and claimed that the changes undermined the “right to strike”.

They argued that the 2022 Conduct of Employment Agencies and Employment Businesses (Amendment) Regulations were unlawful.

Ministers disputed the unions’ claims.

A judge on Thursday ruled against the Government and quashed the 2022 regulations.

Mr Justice Linden had considered arguments from lawyers representing the TUC, unions and the Government at a High Court hearing in May.

The decision to implement the regulations was taken when Kwasi Kwarteng was business secretary, lawyers said.

Lawyer Richard Arthur, who represented the TUC and unions, hailed a “significant victory for the entire trade union movement”.

TUC General Secretary Paul Nowak said the ruling was a “badge of shame for the Conservatives”.

“This is a significant victory for the entire trade union movement and preserves a vital safeguard in ensuring the right to participate in industrial action is effective,” said Mr Arthur, who is based at Thompsons Solicitors.

“The judgment makes clear that the then Secretary of State had a staggering disregard to his legal obligations when introducing legislation that enabled employers to engage agency workers to cover the duties of striking workers. He was driven solely by a political ideology to meet a self-imposed deadline to implement the regulations in the face of mounting industrial action across the country.

“He took this decision notwithstanding advice he received that it was likely to be counter-productive to the problem he wanted to address and was being rushed through without regard for the duty to consult, which was a fundamental legal requirement.

“This is bad law-making made ‘on the hoof’ and the court has rightly held the Government to account”.

Mr Nowak said: “This defeat is a badge of shame for the Conservatives, who have been found guilty of breaching the law.

“Bringing in less-qualified agency staff to deliver important services risks endangering public safety, worsening disputes and poisoning industrial relations.

“The Government railroaded through this law change despite widespread opposition from agency employers and unions.”

Unite general secretary Sharon Graham said: “This is a total vindication for unions and workers. The Government’s decision to allow employers to recruit agency workers to undermine legal strike action was a cynical move to back their friends in business and weaken workers’ legal rights to withdraw their labour.

“It was entirely counterproductive as, rather than weaken industrial action, it has hardened attitudes and unnecessarily extended strikes. This ill-thought out, divisive legislation must be consigned to the dustbin of history.”

Barrister Daniel Stilitz KC, who led the Government’s legal , had told the judge that the law change was “modest”.

Mr Justice Linden said, in a written ruling published online, that Mr Kwarteng had “precious little information” when he made the decision on the regulations.

The judge said he was “not sufficiently” interested “even” to ask to see an analysis by a civil servant.

He said the “decision was to proceed at exceptional speed” despite the civil servant’s concerns about the “effect on Parliamentary scrutiny” and “without any further consultation at all”.

“This was not an all or nothing decision: there could have been a shortened consultation, and/or one with a more limited group of consultees,” said the judge.

“There is no sign that this option was even considered.

“This was despite the lack of an impact assessment at the time of the decision, and despite the evidence available to Mr Kwarteng being that the measure would have negligible beneficial impact in the short term and, quite possibly, an adverse impact on the Government’s ability to settle ongoing industrial disputes.”

A spokesman for Thompsons Solicitors, which represented the TUC and unions, said after the ruling: “Mr Justice Linden found that the government had acted unlawfully in not consulting with the unions.”

A Department for Business and Trade spokesperson said later: “We are disappointed with the High Court’s decision as we believed the decision to repeal the ban on agency workers covering strikes complied with our legal obligations.

“The ability to strike is important, but we maintain there needs to be a reasonable balance between this and the rights of businesses and the public.

“We will consider the judgment and next steps carefully.”