Trade union Unison has been warned to clarify future claims after a watchdog found three press adverts about proposed pension reforms were not "sufficiently substantiated".
The national press ads which appeared in November were headlined: "£413 worse off a year at work £1,069 worse off a year in retirement," "£68 less every month in her pay £18,000 less in her pension fund" and "£597 worse off today £1,275 worse off tomorrow".
One complainant, Matthew Hancock MP, said the ads misleadingly implied that proposed changes to pensions would come into force, whereas discussions were ongoing, and the financial claims were also misleading and unsubstantiated.
Unison said the aim of the campaign was to demonstrate that those involved in strike action were ordinary people who would have to make a difficult decision to lose a day's pay in order to demonstrate against the proposals.
Unison said the ads related to case studies of actual members, whose names might have been changed.
They said the subject and detail of pension issues was complex but care had been taken to ensure the accuracy of the figures stated in all three ads. The figures and references had been checked rigorously by their in-house pension professionals, adding that pension experts would also check figures and wording before similar ads were produced in future.
But the Advertising Standards Authority (ASA) said: "We considered readers were likely to understand that the changes the ads referred to were under continued discussion at the time they appeared.
"We noted, however, that the ads made particular reference to financial sums and that those absolute claims were not qualified with any explanation of how they were calculated. We considered the claims were likely to be interpreted as relating to the costs the women in the ad would incur if the changes proposed by the Government came into effect."
It ruled that the ads must not appear again in their current form, adding: "We told Unison to ensure they were in a position to substantiate future objective claims."