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Ministers have rejected claims a referendum on the Westminster electoral system cannot be held on the same day as parliamentary elections in Scotland.

A Labour peer said "combining" the two votes on 5 May 2011 was not possible because the rules of the Scottish Parliament did not permit this.

Justice Minister Lord Wallace admitted current rules would have to be amended by passing new legislation.

But he said Scottish election officials saw "advantages" in a combined date.

The date chosen for a vote on replacing the current first-past-the-post system for electing MPs at Westminster with the alternative vote system has caused widespread anger among Labour, the Conservatives and nationalist parties in Scotland and Wales.

The latter are concerned it will "overshadow" elections for devolved administrations and risk a repeat of 2007 when simultaneous elections for the Scottish Parliament and council elections caused chaos and thousands of spoilt ballots.

Meanwhile, Labour MPs and many within Tory ranks say it will distort the result since voters in London - where there are no other elections that day - will have less incentive to take part.

Raising the issue in the Lords, Labour peer Lord McAvoy said the date was not "based on the needs of the country but on the needs of a shabby collaboration between two political parties".

He suggested the Interim Electoral Management Board for Scotland believed that the two votes could not be held simultaneously under current Scottish Parliament rules.

Lib Dem peer Lord Wallace, Advocate General for Scotland, said that under the provision of the Scotland Act - which established the devolved Parliament - elections to that Parliament could not currently be held on the same day as a referendum.

However, he said the electoral body did not believe this would be an impediment to this happening and "encouraged" the UK government to amend orders for Scottish elections to allow a combined poll.

He said this would be done as part of legislation this autumn which MPs must pass to give the go-ahead for a referendum.

Deputy Prime Minister Nick Clegg, who has led calls for a referendum next year, has said that holding it at the same time as devolved elections will increase turnout and save money.

He has said he "struggles to understand" why doing this would confuse voters in Scotland and Wales.

Lord Wallace said Tom Aitchison, the convener of the elections body, had concluded that a "formal combination of polls will have many advantages for both the voter and the electoral administrator".

But Labour peer Lord Faulkes said the UK government should "honour" the position of the Scottish Parliament if it was opposed to a 5 May date.

Lord Wallace said there were "many examples" around the world of multiple votes being held on the same day and it was an "insult" to the Scottish people that they could not "cope" with this.

The leadership of the Conservatives and Lib Dems are expected to campaign on different sides of the argument when the referendum is held. However, both parties' MPs will be expected to vote for legislation establishing the referendum.