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David Cameron has been urged to rethink plans for holding a referendum on voting reform at the same time as the Scottish Parliamentary election.

Scotland's First Minister Alex Salmond said in a letter to the prime minister that holding the votes on the same day could "overshadow" devolved issues.

Deputy Prime Minister Nick Clegg has acknowledged there were "concerns" about holding the referendum on 5 May.

However, he has said the proposal would save up to £17m.

Voters in Scotland, as well as those in Northern Ireland and Wales, are due to go to the polls on 5 May to elect politicians to devolved administrations.

Mr Salmond told Mr Cameron his proposed date to hold the referendum on adopting the alternative vote (AV) system for Westminster elections did not meet the Tories "respect agenda" with the rest of the UK.

Mr Salmond's letter said: "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 would be confused and mixed messages would come from London-based media, he argued.

Holyrood voted in 2009 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 said that holding the votes on the same day would save cash and prevent voters having to make "multiple trips" to the polling station.

He added: "To say that this will confuse voters is to underestimate the intelligence of the Scottish people.

"Many countries routinely hold elections and referendums on the same day. I am confident that Scotland can do the same.

"There will be ample time in the election campaign to debate all the issues facing Scotland, and for Scots to make their judgment about who is best placed to deal with them."

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.