Prime Minister David Cameron was today 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.
Mr Salmond's letter continued: "When you visited Edinburgh shortly after your election, you placed great emphasis on what you called 'the respect agenda' between the UK Government and devolved administrations.
"It is not clear how the decision to hold the AV referendum on the same date as our elections, and to do so without any prior consultation, fits into the spirit of that framework."
SNP and Labour politicians have also voiced concerns about the UK Government's plan to hold the 2015 general election on the same day as the Scottish Parliament vote that year.
Lib Dem Scottish Secretary Michael Moore has offered to have talks on the potential clash with Mr Salmond and the Scottish Parliament's Presiding Officer, Alex Fergusson.
Scottish elections run on a four-year fixed term while UK general elections would follow a five-year term, under wider reform planned for Westminster.
The changes, if passed, would lead to clashes in election dates once every 20 years.Reuse content