The electronic scanning equipment will be able to count two-and-a-half ballot papers a second.

That will mean Londoners will be able to know who their new mayor is the morning after the ballot, compared with the three days a manual count would take.

The system, which was used last year in elections in Bosnia-Herzegovina, will be given a dry run in mock elections held in the new year.

'Today's technology'

Environment minister Lord Whitty said: "The election will provide an excellent opportunity to introduce a more up-to-date method of counting votes, more in keeping with today's technology.

A manual count would take three days

"This will be the first time such a modern system of counting has been used in an election in the United Kingdom."

The elections for the mayor and the assembly will be held on 4 May using two forms of proportional representation.

Lord Whitty, replying to a parliamentary question, said: "We estimate that a manual count in next year's London elections could take up to three days before a result is known.

"With the new equipment, a result will be known by the next morning.

"For an election in the new millennium it is only right that we use the latest technology to speed up the process, as long as it can guarantee an accurate count and maintain public faith in the secrecy and integrity of the election process."

Traditional cross

Voters in next May's elections will still be required to put a cross by the names of their preferred candidates.

But they will have to remember not to fold their ballot paper in the traditional way as the machine can only read an unfolded paper.

Notices in every polling booth will tell voters to put their papers in the box with the blank side upwards to ensure privacy.

The election will be fought by millionaire novelist Lord Archer for the Tories and businesswoman Susan Kramer for the Liberal Democrats.

The Labour has yet to select its candidate who will fight next May's election.

Former health secretary Frank Dobson, ex-GLC leader Ken Livingstone. Glenda Jackson MP and broadcaster Trevor Phillips all hope to be selected to become Labour's candidate.