The controversial transatlantic trade deal set to be agreed this year would mean that privatisation of elements of the NHS could be made irreversible for future governments wanting to restore services to public hands, according to a new legal analysis.

The legal advice was prepared by one of the UK’s leading QCs on European law for the Unite trade union, which will reveal on Monday that it has been holding talks with the government about the Transatlantic Trade and Investment Partnership (TTIP) deal between Europe and the US.

Unite believes the government has been keeping Britain in the dark over the impact of the deal and argues the NHS should be excluded from the trade deal. The government dismissed the idea that TTIP poses a threat as “irresponsible and false”.

TTIP would give investors new legal rights, which extend beyond both UK and EU law as well as NHS contracts, according to Michael Bowsher QC, a former chair of the Bar Council’s EU law committee who was tasked by Unite to prepare the advice.

Bowsher said he had concluded that the deal poses “a real and serious risk” to future UK government decision making regarding the NHS.

“We consider that the solution to the problems 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.

In the most explicit warning from a figure of prominence about the potential threat to the NHS from the deal – which may yet prove a rallying point for leftwing out campaigners in the EU referendum – he also warned that it could allow private companies with links to NHS contracts to win higher levels of compensation through bypassing domestic courts.

He also said that TTIP’s procurement rules could force the NHS to contract out services it wants to keep in house or spin off them off as “mutuals”

Unite, which has already held a series of high-level meetings with officials from the Department for Business, Innovation and Skills (BIS) and with the minister for trade, Francis Maude, will present Bowsher’s advice to the government on Tuesday.

Unite’s assistant general secretary, Gail Cartmail, said: “The UK government has no right to allow EU bureaucrats to negotiate away our ability to control the future of our NHS. David Cameron has the power to exclude the NHS from the trade deal – he must act and prevent the irreversible sale of our NHS.

Campaigners have already accused the government of blocking access to legal advice showing its impact on the health service and the extent to which private health companies could sue the government using a secret tribunal system if a Whitehall policy change were to hit their profits.

Unite said that Maude has admitted to having legal advice concerning the TTIP’s potential impact on the NHS but has blocked a freedom of information request to obtain the advice, citing “legal professional privilege”.

A BIS spokesperson said: “The NHS is under no threat whatsoever from the TTIP deal or any other trade and investment agreement. It cannot force the UK to privatise public services or prevent it from regulating in the public interest and any suggestion to the contrary is both irresponsible and false.

“It will remain up to the UK government and devolved administrations to decide how to run publicly funded health services, including whether private companies should be involved. Where a service has previously been provided by a private provider, this is not irreversible.”