The NHS will become "completely unrecognisable" if Britain remains in the European Union, a former Labour foreign secretary has said.
Lord Owen said he had "no doubt" that the health service would become more fragmented under agreements set in Brussels that the UK would be unable to ignore.
The peer, who campaigned as health minister to remain in the European Economic Community in the 1975 referendum, insisted the "momentum" in the EU meant the health service would be "ever more involved with EU competition, procurement law and marketisation".
Launching Vote Leave's Save our NHS campaign in London, he said: "That is the direction of travel. It is to make it completely unrecognisable."
Lord Owen argued that a Brexit would free the UK from the proposed Transatlantic Trade and Investment Partnership (TTIP) between the EU and US, which critics fear will lead to American health providers targeting the NHS.
The peer insisted severing ties with Brussels would allow the UK to take back control of the NHS and protect it from outside competition.
He said: "We are agreed in Vote Leave that whatever our political views on the present marketisation of the NHS, these decisions should for the future be for the UK Parliament and devolved administrations to take.
"It should not be for the European Commission nor the European Parliament."
He added: "It involves your families, it involves your homes, it involves your life. This is not a decision for a remote bureaucracy in Brussels."
Anti-TTIP campaigner Caroline Lucas dismissed claims that quitting the EU would protect the NHS.
The Green Party MP said: "TTIP does post a threat to the NHS, but leaving the EU won't protect us from damaging trade deals.
"If we were to leave then the Tories - who are cheerleaders for TTIP - would pass equally dangerous bilateral trade deals which threaten our most valued public service. British MPs and MEPs have a chance to stop TTIP - it's down to them to act in the NHS's best interests and reject any deal which threatens our health service.
"The greatest threat facing our NHS comes in the form of pro-privatisation politicians passing damaging domestic legislation which cripples our health service."
The Government has insisted the trade deal does not threaten the UK's health service, but offers trade openings for British business.