Every worker wanting to join a strike picket could be forced to identify themselves to police, carry a letter of authorisation and wear an armband, under proposed reforms to trade union laws that could be in breach of international agreements, human rights groups have warned.

The Government’s plans for new legal hurdles making it harder for union members to take strike action are described as a “major attack” on civil liberties in the UK, in a joint statement by Liberty, Amnesty UK and the British Institute of Human Rights.

Accusing the Government of “seeking to undermine the rights of all working people,” they claim the measures will discourage ordinary people from coming together to protect their jobs, surrendering more power to employers, and weakening the quality of their working lives.

The Trade Union Bill has already come in for heavy criticism after the Government published its plans without conducting an impact assessment. Campaigners are stepping up their efforts against it as public consultation closes on 9 September.

Among the measures most concerning to the civil liberty groups are plans to extend strict new requirements on picketing supervisors to everyone present at a strike protest.

The original plans set out in the Trade Union Bill would have required only a picketing supervisor to hand over his name and contact details to police, to wear an armband, and to carry a letter of authorisation issued by the union.

But the Government’s consultation raises the prospect of going even further and requiring all those present at a picket having to do the same. Liberty has described it as “authoritarian” and would discourage workers from joining pickets in fear of being blacklisted by employers and police.

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The consultation asks: “Are there other practices that should be directly legally enforceable - for example, training or a requirement for all pickets to be properly identifiable in the same way as the supervisor?”

Liberty said that demanding that all picketers hand over contact details to police would signal a “hark back to historical problems” between trade unions and the authorities.

This was a reference to widespread allegations that the police and security services previously passed on names and contact details of trade union members to a database that firms consulted before offering people jobs.

“With a history of blacklisting it’s entirely understandable why trade union members don’t want to identify themselves to the police and give the police their phone numbers,” Sara Ogilvie, policy officer at Liberty, said, warning that strike action would soon cease to exist in the UK if the proposals went ahead.

The proposed Bill would also constitute a “clear breach” of Britain’s obligations under international labour standards, experts at Liberty has claimed – despite the Bill containing a statement from the Business Secretary Sajid Javid on its front cover insisting the provisions are compatible with the European Convention on Human Rights.

Proposals such as the requirement that unions count abstentions in ballots as ‘no’ votes that would also be deemed illegal under International Labour Organisation standards, of which British is a member, Liberty claims.

The Government wants to curtail strike action by requiring a turnout of at least 50 per cent of union members for industrial action to be legal. There would also be an additional requirement in “important” public services that strikes be supported by at least 40 per cent of all those eligible to vote.

This would mean that any worker who abstains or forgets to return their ballot paper would be deemed to be opposing the move – contravening ILO standards. “We definitely think the Government is playing fast and loose with its international obligations,” said Ms Ogilvie.

The statement from Liberty, Amnesty UK and the BIHR concludes: “Taken together the unprecedented measures in the Bill would hamper people’s basic rights to protest and shift even more power from the employee to the employer.

“It is hard to see the aim of this bill as anything but seeking to undermine the rights of all working people. We owe so many of our employment protections to Trade Unions and we join them in opposing this bill.”

Opponents of the Government’s reforms point to recent examples of restaurant chains pocketing tips left by customers to cover “administrative costs” as examples where trade unions are essential.

Ms Ogilvie says: “Strikes are actually a tiny, tiny part of what trade unions do, but they are vital for acting as a stick to give trade unions and employees the opportunity to go to employers and say ‘look: this is something we need to talk about, this is something we need to get sorted out’ and let the employer know that the threat of a strike is always there.

“If you take that threat away, people who are not getting paid the minimum wage, people who are not getting their tips because firms are taking them directly out of the till; they’re going to have no way of enforcing their rights, which is completely at odds with what the Conservatives are saying about being the party of the workers.”