Prime Minister David Cameron has been urged to re-think plans for a referendum at the same time as elections to devolved administrations.
Scotland's First Minister Alex Salmond spelled out his anger in a letter to the Tory leader, arguing that it would confuse voters and overshadow devolved issues.
Deputy Prime Minister Nick Clegg has acknowledged there were "concerns" about holding the referendum on the alternative vote system on May 5, but said the proposal would save £17 million.
Voters in Scotland, Northern Ireland and Wales will be going to the polls that day to elect politicians to devolved administrations.
Mr Salmond told the Prime Minister his plan did not meet the Tories' much-vaunted "respect agenda" with the rest of the UK.
He added: "I believe that your proposals to hold a referendum on the same day undermines the integrity of the elections in Scotland, Wales and Northern Ireland.
"These elections are of profound importance to our citizens and I believe they have the right to make their electoral choices for the respective devolved chambers without the distraction of a parallel referendum campaign on the UK voting system."
Agendas will be confused and mixed messages will come from London-based media, he argued.
Holyrood voted last year to separate national and local government elections after the mixed 2007 Scottish poll, which was marred by thousands of spoiled ballot papers.
If the plan goes ahead, voters will be asked to handle three ballot papers in Scotland: two for devolved elections, combining first-past-the-post and a form of proportional representation, and the referendum.
