The Trades Union Congress, representing 14 health unions, said its research showed that health staff in England were "donating" £1.5 billion worth of unpaid overtime every year.
Unions said that by 2015/16 NHS staff would have had their pay capped for six years.
Pay was frozen in 2011 and 2012, and limited to 1% last year.
TUC general secretary Frances O'Grady said: "Every year, hard-working health service staff put in many extra hours which they don't get paid for.
These efforts save the government a hefty £1.5 billion a year, but even though this unpaid overtime is effectively keeping the NHS going, health service employees increasingly feel that the government is taking them for granted."