The deadline for court challenges to last month's local and European elections comes at midnight on Friday.
Postal voting trials have raised concerns
The increase in postal voting raised at the polls new fears of fraud in some areas of England.
The High Court has received four complaints and another two were expected on Thursday afternoon.
They include claims of postal-voting fraud in Bordesley Green in Birmingham, part of one of the areas holding Westminster by-elections this month.
The Tories want special observers to oversee this poll and the by-election in Leicester South.
The Liberal Democrats have lodged a court petition over fraud allegations in the Aston ward of Birmingham.
John Hemming, the Lib Dem group leader on the city's council, has claimed voters were put under pressure to vote a certain way.
A councillor was allegedly seen sorting through ballot papers in a car park and a postman was offered £500 for his sack of postal votes, he says.
An Independent candidate in the European elections for eastern England has also lodged a complaint over the voting in the European elections.
Jim Naisbitt complained that postal votes should only be allocated to those who could prove they needed them. He also argued the design of the ballot paper was confusing to voters.
In Hull, one of the areas trialling all-postal voting, Independent candidate John Considine is making a challenge after he lost his council seat by just seven votes.
Chris Jarvis, leader of the Independent group on the council, said many voters in the city's Derringham ward had been sent the wrong ballot papers and not received replacements during the election campaign.
He told BBC Radio 4's Today programme: "We haven't received as yet any accurate figure giving the total number of people who actually voted in any kind of election.
"In order to do a proper count one has to know how many people voted in the first place."
Mr Howard listed a catalogue of fraud claims at PMQs
Once the petitions are accepted, the complainants have 28 days to ask the court for a directions hearing, which will determine how best to proceed with the cases.
The hearings will assist both sides to decide what evidence must be presented.
Michael Howard revealed at PMQs that the Conservatives have asked the Electoral Commission to oversee the upcoming parliamentary by-elections in Leicester South and Birmingham Hodge Hill.
Tony Blair has promised to look at the idea.
The commission says it has a duty to report on general elections, European parliamentary elections and pilot schemes, but no remit on Westminster by-elections.