Parts of the NHS might have to be privatised if the controversial TTIP trade deal between the European Union and the United States is signed, according to a legal expert.

The advice, which was commissioned by the Unite Union, was drawn up by Michael Bowsher QC, a former chair of the Bar Council’s EU law committee, the Guardian reported. The Government has also received legal advice on the issue but has refused to publish it despite freedom of information requests.

The Transatlantic Trade and Investment Partnership (TTIP) could become an issue in June’s EU referendum, particularly among left-wing voters.

Mr Bowsher said TTIP, which is expected to be finalised this year, might force the NHS to contract out some services to private firms. It would also make it difficult to hand back contracts to the NHS.

The lawyer, in a report to be published on Monday, concluded there was “a real and serious risk” that the Government would not be able to make key decisions about the NHS because ministers would be constrained by the trade deal.

The solution, he said, was “for the NHS to be excluded from the agreement, by way of a blanket exception contained within the main text of TTIP ”.

Gail Cartmail, Unite’s assistant general secretary, told the Guardian: “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.”

However the Department for Business, Innovation and Skills, which is leading on the TTIP negotiations, 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.