The first ministers of Scotland, Wales and Northern Ireland have urged David Cameron not to hold an EU referendum in June, saying it will "distract" from devolved elections taking place in May.
Nicola Sturgeon, Carwyn Jones and Arlene Foster have written to the PM to urge him to defer the date.
They say a June date will "confuse" the process and make it hard for them to campaign effectively in the referendum.
Mr Cameron told MPs that voters were able to differentiate between polls.
David Cameron has pledged to hold the referendum by the end of 2017 but said he wants to get on with it and is reported to favour a poll on 23 June.
However, the exact timing hinges on whether EU leaders agree to the draft package of reforms published on Tuesday at a summit next month, which would pave the way for a poll at the end of June.
In their letter, also signed by Northern Ireland deputy first minister Martin McGuinness, the three political leaders say it is vital that voters are properly informed about the issue at stake in the referendum and this could be compromised if the campaign overlaps with that for elections to the Scottish Parliament, Welsh Assembly and Stormont Parliament on 5 May.
"We believe that holding a referendum as early as June will mean that a significant part of the referendum campaign will necessarily run in parallel with those elections and risks confusing issues at a moment when clarity is required," they write.
Analysis by Nick Eardley, BBC Scotland's Westminster correspondent
Opposition in Scotland, Wales and Northern Ireland to an early referendum has been building in recent weeks.
There is concern that a date just weeks after the 5 May election would mean the campaigns become confused or there wouldn't be time to properly consider the European issue.
It is interesting to note the Labour Welsh First Minister Carwyn Jones has signed this letter.
Nationally his party favours a June vote - Alan Johnson, the leader of the Labour In campaign said yesterday his party would not stand in the way of a 23 June referendum. And last night, Scottish Labour's leader Kezia Dugdale told BBC Scotland she was happy for the vote to be held then.
But expect continued pressure from devolved governments and administrations on this issue. Even if David Cameron can persuade other European leaders of his renegotiation plans, it's possible he will face significant opposition domestically on the date.
"Furthermore, it will be virtually impossible for the political parties in our respective territories to plan effectively for, and where appropriate work together on, the referendum campaign while our own elections are in progress.
"We believe that the European Referendum is of vital importance to the future of the whole United Kingdom and the debate leading up to it should, therefore, be free of other campaigning distraction."
Raising the letter during Prime Minister's Questions, the SNP's Angus Robertson said the PM should show "respect" for the voters in Scotland, Wales and Northern Ireland and make a commitment not to hold the poll in June.
Mr Cameron said no date had been agreed but he could guarantee that the referendum would not be held within six weeks of devolved elections, which he claimed had been asked for by former Scottish first minister Alex Salmond.
"I respect the electorates of England, Scotland, Wales and Northern Ireland on the basis that I think people are perfectly able of making up their minds in a local election or Scottish Parliamentary election and then a period of some weeks later making up their mind all over again on the vital question of the European Union," he said.
"No date has been fixed and there must be a six-week gap but, frankly, I think he is looking for things to complain about."