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Holding a UK referendum on voting reform on the same day as the Holyrood election has been branded an "act of disrespect" by the Scottish government.

Deputy Prime Minister Nick Clegg is expected to announce that the referendum on the alternative voting system will be held on 5 May 2011.

A UK government source said holding the referendum on the same day should improve voter turnout.

But the Scottish government warned the move could cause voter confusion.

The referendum on whether to ditch the current first past the post system would co-incide with English local and Scottish Parliament and Welsh Assembly elections.

Speaking on ³ÉÈËÂÛ̳ Radio's Good Morning Scotland programme, Scottish External Affairs Minister Fiona Hyslop said the UK government had not consulted with the devolved administrations over the proposal.

She added: "It would be an extraordinary act of disrespect for a referendum on the voting system, which none of the mainstream parties in the UK actually support, to be held on the same day as the Scottish elections without any consultation with the Scottish government.

"It is important that we see action, not just warm words on respect. I think this is an example of the British government just not working for Scotland.

"I think this announcement shows a bit of coalition chaos, where the parties in London seem quite prepared to have a referendum on a voting system they don't really support, but not have a referendum on independence to decide the powers the Scottish people could have over issues that could really affect them, such as the economy and benefits."

Ms Hyslop said holding council and Scottish parliament elections on the same day in 2007 had led to "chaos".

The main UK parties have opposed Scottish government plans to hold a referendum on Scottish independence.

But Scottish Labour leader Iain Gray said Ms Hyslop was right to complain about a lack of respect emanating from London, and accused the coalition UK government of acting in a "high handed way".

He said: "It's a bad idea and for once I agree absolutely with the Scottish government, I think it is quite an extraordinary example of disrespect.

"As far as we're aware there has been no discussion, no consultation and I think Fiona Hyslop is quite right.

"The Tory-Lib Dem coalition make great play of coming to the Scottish Parliament, meeting with the first minister and other parties, plenty of warm words but when it comes to a substantive issue like this we hear about it on the radio."

Under the proposed alternative vote system, candidates are ranked in order of preference and anyone getting more than 50% in the first round is elected.

If that does not happen, the candidate with the fewest votes is eliminated and voters' second choices are allocated to remaining candidates. This continues until a winner emerges.

The Liberal Democrats are set to campaign for the new system, but Conservative coalition partners will oppose it.

The ³ÉÈËÂÛ̳ understands the official announcement is to be made by the government in the next few days.