A dramatic rise in postal votes has raised concerns about potential fraud and logistical problems.
Postal vote applications have risen dramatically
The main parties are accused of asking voters, especially in marginal seats, to send post vote applications to them. They say they are within the rules.
Tory leader Michael Howard said Labour should be ashamed of ignoring calls for tighter controls, but Labour said the number of fraud cases was small.
The row follows fraud scandals in local elections in Birmingham and Blackburn.
A survey in The Times found that postal vote applications overall had almost trebled since 2001. In some key marginals they had risen by almost 500%.
RISE IN POSTAL VOTING IN KEY MARGINALS
Cheadle: 485% to 8,226
Dorset South: 192% to 6,557
Thanet South: 219% to 1,129
Dorset Mid and North Poole: 318% to 4,306
Rugby and Kenilworth: 207% to 6,847
Norfolk North: 163% to 6,323
Weston-super-Mare: 240% to 6,323
Braintree: 333% to 10,000
Taunton: 282% to 11,700
Orpington: 246% to 6,429
Mr Howard said the government had left the system open to abuse by failing to put implement some of the safeguards suggested by the Electoral Commission.
He said: "If we had had our way, the recommendations would have been accepted and implemented and we would not have a voting system fit for a banana republic, which is what a High Court judge said is the present position."
Questions were raised over the Tory clearing house for applications in Dartford and a similar Labour unit in Newcastle. The Liberal Democrats say they have only taken applications on a local basis.
Mr Howard said his party were "complying with the rules in every respect".
A Labour spokesman said its postal clearing house did not handle any votes and was used only to send applications on to the relevant local authority.
Lib Dem president Simon Hughes said the idea of parties having a national centre for collecting postal votes was "extraordinary".
But he admitted: "Traditionally, we have said within the rules that if people haven't got a postal vote and they want one, they can either apply direct to their local town hall or if they return something to us we'll post it straight on."
The Electoral Commission code of conduct does allow parties to send out and receive applications for postal ballots, although it says they should not handle the ballots themselves.
The Electoral Reform Society said that although this was within the law, it was "bad practice".
ERS chief executive Ken Ritchie said the commission would have to review the effect of postal voting and issues of fraud after the election.
Culture Secretary Tessa Jowell defended the postal voting system, telling BBC News only six of 17,000 candidates in last year's council elections had been found guilty of fraud.
"I don't think you can conclude from what is a relatively small number of very serious cases that the whole system of postal voting is somehow corrupt," she said.
BBC political editor Andrew Marr said the row over postal voting was potentially "very serious indeed".
"There are real worries about the way that the parties are handling large amounts of votes," he said.
"We could see the situation after the election result is declared, where in closely-fought marginal seats, there are suspicions and accusations and doubts about whether those results were fair".
The government has invited international observers to monitor the general election, and the Office for Democratic Institutions and Human Rights in Warsaw are due to reply within 48 hours.
A small team of monitors could look at every aspect of the election, including the impact of postal voting.
A judge found fraud was "widespread" in the 2004 local elections in Birmingham and six Labour councillors were 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.
Both areas have seen a massive rise in applications, from 16,000 to 53,000 in Birmingham, and from 7,603 to 20,351 in Blackburn.
The number of postal vote applications in Cheadle, Greater Manchester, increased by 485% to more than 8,000.
The seat is held by the Liberal Democrats, with a majority of just 33.