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

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

A High Court judge ruled in their favour on Thursday.

TUC general secretary Paul 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.

“The courts even found ministers ignored evidence that the measure would be counterproductive.

“This is the same reckless approach behind the anti-strike Bill, which has faced a barrage of criticism from employers, rights groups and international bodies and which has been amended by the House of Lords on three separate occasions during parliamentary ping-pong.”

PCS general secretary Mark Serwotka said: “This latest verdict by the High Court is a further litigation victory for PCS against the Government and a further indication of how they are prepared to ignore the law to pursue their war against workers.

“It was obvious from the outset that the Government had failed to fulfil its obligations on consultation.

“Instead of accepting that and following a proper process, they chose to double down and waste taxpayers’ money on defending the indefensible.

“This legislation has been condemned almost universally, including by employer organisations who warned that it would worsen industrial disputes.

“The Government refused to listen.”

Lawyer Richard Arthur, who is based at Thompsons Solicitors and represented the TUC and unions, said: “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.

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

“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.”

Recruitment and Employment Confederation chief executive Neil Carberry said: “This decision comes as no surprise. The law required a level of consideration and consultation that the rushed, politically driven process of last year simply did not allow for.

“Government must now act swiftly – while the process has been ruled unlawful the regulations remain on the statute book, putting agency workers, agencies, and client businesses into a world of uncertainty about what the law now is.

We are proud to have stood with the other trade unions and the TUC to challenge these changes legally, and we will continue to do so in all those other areas, including minimum service levels, to ensure a level playing field for workers here in the UK

“We encourage the secretary of state to act swiftly to restore the situation that was in place before this ill-judged change was made.”

Mick Whelan, general secretary of the train drivers union Aslef, said: “We are proud to have stood with the other trade unions and the TUC to challenge these changes legally, and we will continue to do so in all those other areas, including minimum service levels, to ensure a level playing field for workers here in the UK.”

Jane Bowen, an employment solicitor at law firm Devonshires, said: “This decision will not be welcomed by employers and the public alike, who have been blighted by industrial action over the past year and with further industrial action planned on train lines next week.

“This judgment puts us back into the pre-September 2022 position, whereby the supply of agency workers to cover striking employees is prohibited, making strikes have a greater impact on the public.

“With public services in particular being the subject of strike action in recent times, this ruling will inevitably impact service levels and only increase the frustration of the public who are affected.

“Such frustration is something the trade unions are seeking to leverage in order to secure the employment terms and protections they are seeking for their members, but ultimately it is the working man and woman who suffers on a day-to-day basis.”

These proposals should never have seen the light of day

FDA assistant general secretary Amy Leversidge said of the verdict: “This is a victory for basic fairness and for the trade union movement.

“The Government tried to railroad through a plainly unjust and unfair measure in an attempt to undermine the fundamental right to take industrial action.

“The judgment found that ministers ignored advice that this served no real benefit for the public and was ultimately unworkable.

“These proposals should never have seen the light of day.”