A ballot of more than 300,000 NHS workers on the Government's controversial pension reforms showed just over half voted against, but in a low turnout of less than 15 per cent.
Unison said the low numbers of staff in England, Wales and Northern Ireland who took part in the ballot reflected the low morale and current "difficult state" of the health service.
Of the 14.8 per cent of workers balloted, 50.4 per cent were against and 49.5 per cent in favour.
Unison said the result showed the pension changes were not being accepted, but equally, there was no mandate to take further industrial action.
Unison members took part in last November's strike by more than a million and a half public sector workers, but will not now join another day of action on May 10.
Christina McAnea, Unison's head of health, said: "The low turnout coupled with the close vote shows there is no mandate to endorse the pensions proposals, but equally no mandate to take further industrial action.
"The turnout is disappointing but in some ways is not unexpected. Our members in health, including nurses, paramedics, occupational therapists, porters, medical secretaries and healthcare assistants, are in the second year of a pay freeze, many face job cuts and increasing pressure at work as well as attacks on their terms and conditions.
"In addition, the Health and Social Care Act is set to cause major disruption across the service. The turnout reflects the low morale and current difficult state of the NHS.
"We need to consider the next steps in the pensions campaign and we will be talking to the other health unions."
Civil servants, NHS and defence staff in the Unite union and employees at the Royal Fleet Auxiliary will take action on May 10 in the continuing campaign against the pension reforms.