The Conservatives have warned of a "nightmare" for voters after ministers announced that next year's elections to English local councils and the London assembly will be held in June rather than May.

Ministers will back primary legislation to bring about the change in the hope that it will help lift turnout from the 37 per cent recorded last week.

The move means that the 2004 English local elections and Greater London Authority elections will now take place on the same day as the European parliamentary elections.

Having faced one "super Thursday" this year, with English local, Scottish parliament and Welsh assembly elections taking place on the same day, the parties now face the prospect of another key test of public support.

The changes will see the local and London polls moved from May 6 to June 10 2004.

The change prompted criticism from the Conservatives, who accused the government of attempting to limit public debate on key issues.

"Moving elections to the same day will prevent a proper debate on the separate issues of Europe and London government. We believe that these are debates the Labour Party wishes to close down due to its record of failure in London and local government," said shadow minister for London Eric Pickles.

But the government said the change would make voting more convenient.

"We want to see a modern and efficient electoral system. A system that is straightforward and convenient for voters. This change is another step towards such a system," said local government minister Nick Raynsford.

And Yvette Cooper, the minister with responsibility for European parliamentary elections, said the move would avoid the need for the public to vote twice in the space of six weeks.

But Pickles said the move would not lead to higher turnout and could confuse London's voters.

"This is going to cause a nightmare for Londoners on election day: voters would be given three different ballot papers, each with a different electoral system, casting five different preferences," he said.

"In the 2000 London assembly elections, a worrying eight per cent of votes were spoilt or blank - it could be far worse with yet more paperwork.

"Low turnout will ultimately not be addressed by initiatives like tinkering with the days of elections. The government would do better to tackle the causes of low turnout - disillusionment with political institutions like the European Union and the lack of autonomy of local government."