Another group of health workers is being urged to reject the government's offer of a three-year pay deal.
The 26,000 members of the Chartered Society of Physiotherapy will be recommended to turn down the offer, worth just over 8%, mainly because it is below the rate of inflation.
Peter Finch, an official of the CSP, said: "The CSP is not opposed to multiyear pay deals in principle, but with current economic uncertainty a below-inflation offer is simply unacceptable for our members.
"This is the first time in 12 years that we have had to hold a ballot with members over pay and that goes to show the level of their anger over this proposed pay deal.
"It's certainly doing nothing to improve the morale and motivation of these hard-working health professionals."
The ballot will close on May 13 after which the CSP leaders will consider the result and confer with other NHS unions before deciding on what action to take next.
Midwives are also being urged to reject the offer by the Royal College of Midwives, although leaders of the Royal College of Nursing are recommending acceptance.
About half a million NHS members of Unison are being balloted with no recommendation.