Allegations of electoral fraud have overshadowed the end of campaigning for the European and local elections.
14m can only vote by post
The claims being investigated by police in Lancashire and Manchester are that some voters have been intimidated into handing over their blank ballot papers
Others have been forced to vote for a particular party, it is claimed.
The Tories say Labour is playing "fast and loose" with democracy by holding large scale post voting trials. Labour counters that turnout has risen.
Greater Manchester Police are probing malpractice claims while the Lancashire force will question 60 people about 170 proxy vote applications.
Deputy Tory leader Michael Ancram claimed that in one case an employer told his staff he would sack them all if they refused to support Labour.
Meanwhile, in Bradford, there are reports from Asian areas of voters handing over their ballot forms to political canvassers.
One man told BBC Radio 4's PM: "They are knocking on doors and saying give us your forms, we'll fill them in for you and we'll post them for you...they are trying to fiddle the elections."
The claims were backed up by Keighley Labour MP Ann Cryer, who said the problem may be down to the way the area's Asian communities operated.
"If one of their leaders or elders comes to the door and asks them to do something, they by and large do it," she said.
The allegations are not being directed at any one particular party, but across the political spectrum.
The Electoral Commission's chairman Sam Younger said he intended to investigate allegations of fraud and intimidation and look at measures to tackle it.
The Conservatives and Lib Dems both opposed Labour's decision to carry out all-postal ballot trials in four regions, and not three as advised by the Electoral Commission.
In the Commons Michael Ancram said: "It's not just about turnout it's about the integrity of the system, it's about confidence, it's about trust, it's about no corruption and it's about people being able to exercise their vote."
Standing in for Tony Blair at prime minister's questions, Deputy Prime Minister John Prescott insisted any malpractice or illegality would be thoroughly investigated after what were, he stressed, pilot schemes.
He added that in areas where the experiment was taking place "the evidence shows that the amount of people participating is considerably higher than it was".
Mr Ancram described this response as showing "breathtaking complacency".
All-postal ballots are being piloted in the North East, North West, East Midlands and Yorkshire and Humber regions.
The process has been subjected to a series of hiccups.
In Bolton the local authority is having to set up two emergency polling stations after 1,000 ballot papers were undelivered.
Another council is reportedly having to deliver ballot packs by hand following production delays.
A Department for Constitutional Affairs spokesman insisted safeguards were in place to prevent fraud.
The spokesman said criminal allegations were a matter for the police.
"The Electoral Commission is writing a report about the pilots and we will be reading it in great detail," he added.
All-postal ballots are being tried out because they are perceived to boost turnout - something that seems to be being achieved.
Indications suggest turnout at 10 June's local and European election is higher than last time in areas with all-postal voting.
Figures from the Department of Constitutional Affairs show that by 8 June 27% of ballot papers had been returned across the pilot areas - compared with a 20% turnout in those areas in 1999.
Elections are taking place in 166 local English and Welsh councils, as well as for London's mayor and assembly.
All UK voters are also choosing their members of the European Parliament.