A 100,000-name petition will be handed in to 10 Downing Street by campaigners against a controversial trade deal being negotiated between the EU and the United States.
Suffragette actress Anne-Marie Duff will today join activists who are pressing the Government to make sure the NHS is not affected by the Transatlantic Trade and Investment Partnership (TTIP).
Campaigners have warned that parts of the health service would be privatised under the deal.
Duff said: "We can't afford to take risks with the NHS. It saves our lives and brings our children into the world regardless of who we are. The NHS makes us all equals.
"The TTIP trade deal puts that at risk because it could make privatisation irreversible. I want the Government to think again and protect the NHS from TTIP."
John Lipetz, of the Keep Our NHS Public campaign group, said: " Well over 140,000 people have signed our petition calling on David Cameron to protect the NHS from the dangerous TTIP trade deal which threatens our health service with irreversible privatisation."
The Government maintains the deal would give a huge boost to UK businesses, cut red tape and create jobs.
A Department of Health spokesman said: "TTIP poses no threat to the NHS whatsoever - it cannot force the UK to privatise public services and any suggestion to the contrary is both irresponsible and false.
"The Prime Minister, the chief US and EU negotiators, the EU Commissioner and MPs from all sides have been clear on this point."