Fears of electoral fraud have grown following revelations over the scale of postal voting in Thursday's poll.

Amid concerns that the system is not secure it emerged on Tuesday that almost two million people across the country have voted by post already, including tens of thousands in marginal seats.

The figures, from a Telegraph survey of 21 seats, suggest almost a quarter of postal voters have already cast their votes.

Returning officers in Dumfries and Galloway, Northampton South, Milton Keynes North East and Hammersmith and Fulham had received over 3,500 postal ballots by April 28 - well up on 2001 totals.

Since the last general election it has been made easier to cast a ballot by post, with voters no longer having to give a reason for why they cannot go to their local polling station.

It is hoped the move will have an upwards effect on participation in the poll but has led to legal objections over security.

A judge recently found that safeguards in Britain "would disgrace a banana republic" following the conviction of Birmingham councillors for fraud in last year's local government elections.

In London the Evening Standard newspaper revealed that there has been a three fold increase in the number of applications for postal votes, although in Hackney this figure is 40 times higher.

The borough has issued 19,456 postal ballot papers in the Hackney South and Shoreditch seat following an all-postal voting experiment in 2002.

The paper also revealed examples of potential fraud in the Bethnal Green and Bow constituency, where 6,600 postal ballot papers have been issued.