The commission says voters must still be able to use polling stations

Sam Younger, chairman of the Electoral Commission, said the European single currency issue was so important that voting had to held at a different time to other ballots.

The commission also called for more people to be able to vote by post in a report out on Tuesday.

Low turnout in recent elections has worried many people, and it fell to 38% at last month's assembly poll.

The commission said postal ballots had significantly increased turnout during trials.

You'd have the confusion...of two people appearing on a platform... against each other in one set of elections and with each other on a referendum issue Sam Younger, Electoral Commission

Mr Younger emphasised the commission's warning against holding a euro referendum on the same day as an election.

Reports had suggested the euro poll was going to be held on the same day as the recent 1 May elections for the assembly, the Scottish parliament and local authorities.

An issue of such "major constitutional significance" should be treated separately, said Mr Younger.

Combining the two would create confusion, according to Mr Younger.

"You'd have the confusion for the electorate of two people appearing on a platform on the same day against each other in one set of elections and with each other on a referendum issue," he said.

Politicians are worried by a general fall in turnout at elections

"We wouldn't like the idea of a major referendum being held at the same time as party-based elections."

In calling for greater use of postal ballots, Mr Younger said people should still be able to vote at traditional polling stations.

But Mr Younger said: "I think certainly postal voting will be extended."

He told political website ePolitix: "The electorate has appreciated having all-postal ballots, I think in particular getting their ballot paper through the post and being able to return it and not having specifically to apply for a postal vote.

Dominant

"So I think it is very much the trend that we'll move in that direction, and it will certainly be, I think, one of the dominant ways of voting over the coming years.

"That is why we need to make sure that the legal underpinning and the security measures are right to make sure it can be done with confidence."

Mr Younger admitted more needed to be done to end "confusion" in Scottish parliament elections.

Proportional representation means people have two votes in that ballot, as they do in Wales.

"There are ways we've been making progress," he said. "But I wouldn't underestimate what a tall order it is to make sure that people are well enough informed to exercise their vote coherently."