Party leaders unite against June EU referendum date
All four party leaders in the assembly have written to the prime minister objecting to the idea of holding the EU referendum in June.
The Labour, Plaid Cymru, Conservative and Liberal Democrat leaders say a vote the month after May's assembly election could confuse voters.
They say a later referendum would give both polls the "respect" they need.
The leaders say a June referendum would also affect election campaigning in London, Scotland and Northern Ireland.
There has been speculation David Cameron intends holding the referendum on 23 June, depending on the outcome of an EU summit in February.
On Monday, the Welsh government said the EU vote should be "sufficiently distant" from the assembly election.
In the letter, published on Wednesday, Labour's Carwyn Jones, Tory Andrew RT Davies, Plaid's Leanne Wood and Lib Dem Kirsty Williams say: "A June referendum date would mean that the campaigning period for the referendum would overlap with the campaigning for the May elections to our national assembly.
"The simultaneous existence of multiple party political campaigns and EU referendum campaigns would in itself pose practical and logistical difficulties, but the greater problem that we anticipate is the potential for confusion as a diverse range of issues is presented to the electorate.
"This is not just a matter of respecting the integrity of the Welsh electoral debate, but of affording the EU referendum campaign the respect it deserves."
Scottish First Minister Nicola Sturgeon has also warned it would be "a mistake" to have the referendum so soon after the devolved elections.
Mr Cameron has pledged to hold the referendum by the end of 2017.