Unite has opened a new front in the battle against a controversial transatlantic trade deal by publishing legal advice which warns of a “real and serious” risk posed to a future government’s stewardship of the NHS.

Britain’s biggest union has seized upon a legal opinion prepared by a senior QC which highlights the risks posed by the Transatlantic Trade and Investment Partnership (TTIP).

Unite has fought a long-running campaign against the treaty, which is stuck in marathon negotiations. and which was designed to cut regulatory barriers between the US and the EU but which has been dogged by fears it will undermine the public provision of the NHS. Last month Sajid Javid, the Business Secretary, refused to release the legal advice produced for his department, claiming ministers need to be able to receive candid guidance privately.

Today Unite publishes a legal opinion by Michael Bowsher, QC, a former chair of the EU law committee of the Bar, which recommended a “specific exclusion” for the NHS, when TTIP is agreed, or effectively an opt-out for the UK to take any measure regarding the “organisation, the funding and the provision” of a public NHS.

“We consider that the solution to the problems which TTIP poses to the NHS – and which is likely to provide the greatest protection – is for the NHS to be excluded from the agreement, by way of a blanket exception contained within the main text of TTIP”, Bowsher said.

Unite described the opinion as “explosive” and will send the text to all MPs.

Gail Cartmail, Unite assistant general secretary, said: “The legal advice is clear, TTIP is a real and serious risk to the NHS. Unless the UK gets robust protection, the US private health industry can override the will of Parliament on matters relating to our health.”

Last year EU trade commissioner Cecilia Malmström indicated she was willing to rework elements of TTIP to tackle concerns over the potential for US companies to take a much greater role in the NHS.

“David Cameron has the powers to prevent EU bureaucrats from handing rights over our NHS to America,” added Cartmail.

“If David Cameron does not use the power to exempt the NHS, they will not be able to plead ignorance when TTIP is used to attack the NHS.”

The EU Commission has claimed the treaty could increase the size of the EU economy by €120bn (£93.5bn) by 2027.