The union warned that the decision on Tuesday by health secretary Alan Johnson to allow the fees could create a ‘two-tier’ health service, with more wealthy patients able to afford better care.
Karen Jennings, Unison head of health, said: ‘The big insurance companies are rubbing their hands together at the top-up announcement this week.
'People will fear that treatment will not be available on the NHS when they need it and turn to insurance companies to fund the gap.
‘It is a very slippery slope.
'UNISON believes that introducing top-ups will create a two-tier health service and undermine the founding principles of the NHS.’