The number of people voting by post is expected to have nearly quadrupled since the last general election.
Applications for postal votes have shot up
The Electoral Commission estimated 6.5m people, or 15% of voters, would have applied for postal votes by Tuesday's deadline. In 2001 it was 1.7m.
There have been concerns about security after post vote fraud scandals in local elections in Birmingham and Blackburn.
A newspaper advertising campaign warning of the dangers and giving advice has been launched.
The BBC's own research, based on responses from 349 of the 646 constituencies, suggested at least 5m postal ballot papers would be sent out.
Some marginal seats have seen steep rises in the number of applications for postal votes.
Parties have been giving voters application forms and in some cases have been collecting and delivering them in bulk to the elections office.
The parties say they want to encourage voters to register, but in Birmingham, election officials say it can delay the process and have asked all parties to stop the practice.
Police are also visiting homes in Birmingham to check applications are "bona fide" and reassure voters, it was announced on Tuesday.
And in a BBC investigation, reporter Adrian Addison found it easy to steal the vote of a businessman in Aldeburgh, Suffolk by printing out a form from the Electoral Commission's website, filling in his details and posting it off.
The voter, Craig MacKay, said: "Amazing - for this day and age with the way we live in a society where we need ID for everything and you just come and take my vote like that - where does that leave me?"
He got his postal ballot back - as well as his standard voting card through the post.
The returning officer in Suffolk said staff at the polling station would notice he had already voted by post when he turned up with his card and he would not have been able to vote twice.
It follows a judge's comments earlier this month that fraud was "widespread" in the 2004 local elections in the city, where six Labour councillors have been forbidden to stand.
In Blackburn, a Labour activist was sentenced to three and a half years' jail after rigging votes in the 2002 local elections.
Meanwhile police and election officials in the Labour-held marginal seat of Bradford, West Yorkshire, are contacting 13 people registered to vote at the six-bedroom home of a Conservative councillor.
Jamshed Khan has resigned the Conservative whip while investigations continue, but says they are all relatives, some of whom are on holiday, and denies any wrongdoing.
The Electoral Commission has said it was disappointed its suggestions to make the system more secure had "not been progressed more rapidly".
The chief executive of the Electoral Reform Society Ken Ritchie said he thought the Birmingham case had put a spotlight on the potential for fraud and meant officials would be more vigilant.
But he told BBC News 24: "Undoubtedly there is going to be fraud, but there's been fraud in every election up until now.
"Fraud is particularly easy with postal voting and if we have a lot more postal voting, it just follows that there will be a lot more fraud."
And Conservative Party co-chairman Liam Fox told BBC News 24: "This is an area where the government seems to have put party advantage ahead of the integrity of the system itself."
Home Secretary Charles Clarke has said everything would be done to prevent fraud, and said guidelines had been sent to police and returning officers.
The Liberal Democrats have said any extension of postal voting should be ruled out "until the major potential sources of abuse have been eliminated".