Two thousand nurses are expected to protest, calling for the one percent cap on public sector pay rises to be lifted.
The Royal College of Nursing (RCN) is warning its members could take industrial action.
The demonstration outside Parliament today, (September 6) is being backed by Lewes MP Maria Caulfield.
The RCN said it will be its biggest ever UK rally in central London.
The union said it follows years of pay freezes and a one per cent cap.
Mrs Caulfield previously worked as a full-time cancer nurse until she was first elected to Parliament in 2015.
Maria Caulfield MP said:
“Having worked as a full time nurse under the pay freeze before my first election, I know the pressures that many nurses are facing in the NHS.
“I have met with the Royal College of Nursing and am delighted to be able to give them a chance to put their views across to other Parliamentarians.
“I would urge people to contact their MPs in these last few days to encourage them to attend the event.”
— The RCN (@theRCN) September 5, 2017
There is speculation Theresa May could lift the cap on public sector pay in England and Wales.
Janet Davies, Chief Executive & General Secretary of the RCN, said:
“The public can see the shortage of nurses for themselves.
“Ministers are significantly out of touch with public opinion.
“They should heed this warning, scrap the pay cap and help to recruit thousands more nurses for a safer NHS.
“Experienced nursing staff are leaving in droves – not because they don’t like the job, but because they can’t afford to stay, while the next generation do not see their future in an under-valued profession.”
“If the Government fails to announce a change of direction in the Budget, then industrial action by nursing staff immediately goes on the table.”
Actor and television presenter Tony Robinson will make a speech at the rally.
Scrap the Cap! Join me and an army of nurses at Parliament Square tomorrow 12:30 Scrap the Cap!!!
— Tony Robinson (@Tony_Robinson) September 5, 2017
The event is taking place between 12.30 and 2.30pm.