The Welsh Assembly is to consult on a proposal to hold both elections on 10 June in a bid to boost turnout. But the move could jeopardise plans for all-postal pilots, as
all-postal ballots cannot be used in joint elections.
Welsh Local Government Association head of strategic policy Steve Thomas warned councils who want to run all-postal pilots at next year's elections might not be able to.
The WLGA added that joint elections cause confusion, add to the administrative burden on staff, and do not increase turnout.
Sue Essex, minister for finance, local government and public services, said voters would struggle to understand why the two elections were being held so close together when ministers had the power to hold both on the same day.
'On balance, I believe the convenience of the voter is of paramount importance and that it is more likely a good turnout will take place for both sets of elections if they are held on the same day,' she said.
However, she welcomed the WLGA's comments: 'I will be putting the Assembly's view to the Electoral Commission - who will be advising the UK government - that Wales is very well placed to operate an all-postal election in combined elections next June.'
Council elections, which were due to take place on 6 May, have already been delayed for a year in order to establish clear and separate mandates for local government and the Assembly.