MILLIONS could already have cast their vote for the May General Election by this time next week.

Postal votes are due to go out from next Wednesday, said the Electoral Commission, the elections watchdog, and, traditionally, those who ask to exercise their democratic right via the postbox do so within 48 hours of receiving their ballot by mail.

Potentially, the last political event many could see will be next Thursday's so-called "challengers' debate" when leaders of the opposition parties - Ed Miliband for Labour, Nicola Sturgeon for the SNP, Nigel Farage for Ukip and Natalie Bennett for the Greens - go head to head on the BBC.

At the last election in 2010 some 5.5m voters voted by post, around one in six of the 29.7m people who voted overall.

Party leaders will be keen to know that postal voters are more likely to use their ballot paper than other voters; at the last election 83 per cent of postal voters cast their ballot compared to 63 per cent who attended a polling station.

A Commission spokeswoman confirmed that postal votes were becoming more popular with electors but explained that just how many would use the method for this election would not be known until the figures came in from all the local authorities following polling day.

In the most recent major ballot - the Scottish independence referendum - almost 800,000 people voted, representing just under 19 per cent.

The most popular ballot in the UK when people used postal ballots was the elections for police and crime commissioners in England in 2012. Although the overall turnout was the lowest in history at just 15 per cent, 49 per cent of those who voted, 2.8m people, did so by post.