Councils have reacted with fury to the Welsh Assembly's decision to hold next year's local elections on the same day as elections to the European Parliament.
Combined elections on 10 June will prevent voters judging local government on its own merits and threaten the integrity of any electoral pilots held on the day, according to the Welsh Local Government Association.
Sir Harry Jones (Lab), leader of the WLGA, said the decision 'lacked consistency'.
'The WLGA was wholly supportive of electoral innovation should elections have remained separate,' he added.
'However we cannot now support untried and untested processes when the scale and complexity of combined elections presents an unreasonable risk and threatens the integrity of the process.'
But first minister Rhodri Morgan said combining the two polls would boost turnout and reduce administrative costs.
'We want a good turnout at both elections and believe that cause would not be well served by keeping them apart,' he said.
Combined elections would be 'an excellent opportunity for a successful electoral pilot across Wales', he said.