Labour MPs have been urged to disregard Theresa May‘s “heart-hearted” offer on workers’ rights after a new legal opinion poured cold water over her plans.

Ahead of a Commons showdown on Ms May’s Brexit deal, top QC Aidan O’Neill has drafted a damning assessment of the prime minister’s proposals to protect workers’ rights, which were unveiled last week in an attempt to win over Labour MPs.

The EU law expert, who was commissioned by the Independent Workers Union of Great Britain (IWGB) and the Transport Salaried Staffs’ Association (TSSA), argued that Ms May’s offer provides no guarantee that future EU standards will be matched when the UK leaves the bloc.

In a 47-page dossier sent to all Labour MPs, Mr O’Neill warned that future parliaments could rip up Ms May’s pledges, and even if parliament did implement all new EU standards, workers could still lose out on key protections under EU laws.

Workers would also lose access to the European Court of Justice and the charter of fundamental rights, he said.

“Brexit involves in effect a wholesale bonfire of the ‘vanities’ which this government evidently regards EU law rights to be,” the legal opinion states.

“And even if it were proposed by the government that it would seek to insert into any withdrawal agreement with the EU a legally binding commitment obliging the UK to match EU standards in terms of workers’ rights and other social protection, this is not an obligation which could be directly enforced or insisted upon by individuals before our courts, as it would be a matter of international law only.”

It comes as Downing Street admitted talks with Brussels were “deadlocked”, prompting speculation the prime minister would delay a Commons vote on her deal on Tuesday.

Ms May had pledged to give parliament a fresh meaningful vote on her Brexit blueprint after it was overwhelmingly defeated in January – but speculation was mounting on Monday over the prospect of another historic defeat.

In an attempt to secure backing from Labour MPs, Ms May vowed that parliament would be given a vote on whether to adopt any new EU rules and pledged to work with trade unions and businesses on any changes to protections.

IWGB general secretary Jason Moyer-Lee said: “This legal opinion is a damning indictment of the prime minister’s non-offer last week.

“Workers, and in particular the precarious workers the IWGB represents, need the full protection of EU employment law. The PM’s half-hearted proposals don’t come anywhere near that protection.”

His concern was echoed by TSSA general secretary Manuel Cortes, who warned that the prime minister was “playing fast and loose on the issue of workers’ rights”.

He said: “Our rights are not poker chips to be played with in the Conservative Brexit casino.

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“Our rights are hard-won and it’s the duty of every Labour MP not to collude with the Tories to get rid of them.

“This clear expert legal opinion only goes to show that May’s offer was never a serious prospect on workers’ rights.”