DAVID CAMERON would be “acting unconstitutionally” by imposing the Trade Union Bill on Wales, according to a “damning” legal opinion published yesterday by the TUC. 

The Prime Minister refuted suggestions last month that the Welsh and Scottish governments could block parts of the Bill, insisting it was “not a devolved matter.”

But an independent assessment carried out for the Wales TUC finds it does infringe on the delivery of devolved public services, such as health, schools and local government. 

The Sewell Convention, which enshrines the autonomy of the Welsh Assembly, states Whitehall must seek consent for laws which affect devolved areas. 

And QC Hefin Rees concludes: “A UK government that proceeded in breach of that important Convention would be acting unconstitutionally."

It could prove crucial when the Welsh Assembly discusses whether to give the Bill legislative consent later this month.

Wales TUC general secretary Martin Mansfield said: “We expect that consent to be withheld — meaning the UK government must exempt Wales from parts of the Bill.

“If they refuse then the UK government will be acting unconstitutionally. They will be attacking the basis of devolution and the will of the Welsh people.”

The speaker of the Scottish Parliament may also be forced to reconsider her ruling that a legislative consent motion was not within the body’s powers. 

The advice was revealed as the anti-strike laws passed their second reading in the House of Lords without a vote. 

Tory business minister and former Tesco boss Baroness Neville-Rolfe claimed the Bill is a  “sensible, proportionate measure that will bring industrial life into the 21st century.”

But in her maiden Lords speech, former Labour MP and deputy Commons speaker Dawn Primarolo warned it was a gift to “unscrupulous” employers like Sports Direct.

“The absence of a trade union means that employees have no voice, no-one to represent them and are unable do it themselves for fear of victimisation,” she said. 

Tory ministers were also criticised for failing to produce the required impact assessment of the Bill — more than five months after it was first published.