The Government is being warned it will be in breach of minimum wage laws if it does not inject £280 million into the NHS by the end of the decade.
Unions representing health workers said within five years the lowest paid could be on less than the statutory minimum.
In their submission to the NHS pay review body, the unions pointed out that workers had suffered a government imposed cap of 1% for years.
The unions warned that the wages of those on the lowest pay scales, including hospital porters, cleaners and healthcare assistants, would fall below the legal minimum by 2021, unless the policy changed.
Christina McAnea of Unison said: "Agenda for Change - the NHS pay framework - has served staff well for many years, but sustained government attacks on health workers' pay pose a threat to its future.
"Capping NHS pay at such low levels is giving hospital trusts across the country a huge headache as they struggle to hold on to experienced staff, who can earn much more working for agencies.
"Limits on NHS pay are also deterring many people from taking up careers in the health service.
"The Government must come up with the cash to increase the pay bands of the lowest paid or it will be breaking its own wage laws.
"If the NHS is to continue to deliver the best possible care for patients, its workforce must feel valued and motivated - and that has to start with fair pay."
Jon Skewes of the Royal College of Midwives said: "We are asking for the NHS pay review body to break the pay cap imposed by government and recommend a fair increase for NHS staff.
"Six years of pay stagnation in the NHS have contributed to the increasing shortages of staff including shortage of nearly 3,500 midwives."
A Department of Health spokesman said: "The current minimum NHS basic pay of £7.80 per hour is greater than the national living wage of £7.20 and we continue to increase funding so NHS employers can meet rises in the national living wage."