Rishi Sunak has been warned his plans to make it harder for public sector workers to go on strike would likely breach the European Convention on Human Rights.
A new report by the Joint Committee on Human Rights, made up of peers and MPs, has urged the prime minister to think again about the controversial legislation.
The proposals, which moved rapidly through the Commons and are now in the Lords, aim to make strikes illegal if unions failed to provide a minimum level of safety to the public
They would apply to six sectors, including health and transport.
The plans have been attacked by trade unions, while Labour has promised to repeal the law if it wins power.
In their report, MPs and peers raise concerns the plan would clash with requirements under Article 11 of the ECHR, which guarantees freedom of association for workers.
The committee, whose report comes as peers prepare to debate the bill again on March 9, has called on the government to reconsider the legislation.
In a 45-page consideration of requirements under European human rights law, the MPs and peers warn that “without the government providing specific evidence establishing a pressing social need for minimum service requirements in respect of each of the very broad categories of service set out in the Bill, compliance with the requirements of Article 11 ECHR remains unclear”.
As well as offering alternative proposals, the committee said the penalties that would be imposed on trade unions for failing to comply with the bill would be “severe”.
It said: “In our view, they may amount to a disproportionate interference with Article 11, particularly in circumstances where the strike does not involve essential services and risks to life and limb.
“The government should reconsider whether less severe measures, such as loss of pay or suspension from work for employees who fail to comply with work notices, could be effective.”
Committee chair and SNP MP Joanna Cherry said the bill needs amending to address some of the “deep flaws”.
She added: “Heavy-handed sanctions are compounded by vague rules that would leave striking workers and unions in confusion as to whether they had been met or not.
“The sectors included in the bill are also ill-defined, risking over-reach into areas only tangentially linked to the maintenance of vital public services. This means the Bill, in our view, is likely to be incompatible with human rights law which provides a right to association and with it, protection for strike action.”
She called on the government to “think again and come back with legislation that better respects the protections guaranteed by the European Convention on Human Rights”.
Deputy Labour leader Angela Rayner said the committee is “just the latest expert body to conclude the government’s shoddy bill is not just unworkable but likely unlawful”.
She added: “It’s time for ministers to go back to the drawing board, not plough on with a dog’s dinner of a policy that will do nothing to resolve disputes and instead risks pouring petrol on the fire.”