General secretary Paul Nowak said the union will be lodging a case at the International Labour Organisation (ILO) because the legislation, which will enforce minimum service levels during strike action, "falls far short" of international legal standards.

Making the announcement on the opening day of the TUC Congress in Liverpool, Mr Nowak said: "The ILO has already slapped down the UK government and ordered it to make sure existing and prospective legislation is in line with ILO standards.

"We believe the Strikes Act falls way short of that. And that's why we have submitted a case to the ILO over these new laws."

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The government took forward the Strikes (Minimum Service Levels) Bill following a year of unprecedented industrial action by hundreds of thousands of workers including nurses, teachers, civil servants and railway staff.

It seeks to ensure a legally mandated level of service across key sectors like the NHS during a strike, and will allow bosses to fire employees who ignore notices ordering them to work on strike days.

Speaking to Sky News' chief political correspondent Jon Craig after making the announcement, Mr Nowak called the bill "an attack on working people and an attack on unions in the midst of a cost of living crisis".

He said the "key priority" in lodging the challenge is to ensure that no member of staff is sacked "for taking a lawful decision" to strike.

Asked if the Conservative government will take any notice of the watchdog, he said the Tories have "played fast and loose with its international obligations", but they will "explore every single route to challenge this legislation".

The union chief pointed to the TUC's victory at the High Court earlier this year over the unlawful use of agency workers during strikes.

He said: "The government has got this wrong, we won't rest until the legislation is repealed."

He conceded "that may require a change of government" as he went on to defend Labour against accusations it has become a Blairite "1990s tribute act".

Mr Nowak was backed by European Trade Union Confederation general secretary Esther Lynch, who said: "Rather than bringing the UK in line with its European partners - these draconian laws will cut it adrift."

She added: "It is already harder for working people in the UK to take strike action than in any other Western European country.

"Now your government wants to restrict the right to strike even further."

The new laws would apply to workers in six industries: health, fire and rescue, education, border security, nuclear decommissioning and transport.

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A government spokesperson defended the bill, saying: "The purpose of this legislation is to protect the lives and livelihoods of the general public and ensure they can continue to access vital public services during strikes.

"The legislation does not remove the ability to strike, but people expect the government to act in circumstances where their rights and freedoms are being disproportionately impacted, and that's what we are doing with this bill."