Yesterday, Liam Fox, the Conservative party co-chairman, accused Labour of trading away the integrity of the electoral system in pursuit of victory. “Their casual acceptance of the corruption of Britain’s electoral practices stands as a monument to the new Labour project,” he said.

Over the past month, political parties have rushed to sign up supporters to vote by post and more than 6.5m people have received postal ballots. However, checks are negligible and yesterday election monitors from the Organisation for Security and Co-operation in Europe began scrutinising the voting system — the first time for a British general election.

In Birmingham, where several Labour councillors were recently convicted of electoral fraud, Ardem Yusuf, a charity worker, said: “You will have a household where the man of the house will vote for everyone.”

In Blackburn, Straw’s Liberal Democrat rival, Tony Melia, has made a formal complaint. “I’ve already had two people complaining to me about certain Labour people asking them to hand over postal votes. Postal vote rigging is happening here,” he said.

There is likely to be an unprecedented number of legal actions challenging the results of Thursday’s election with losing candidates alleging impropriety and fraud.

Yesterday one voter in Charles Clarke’s constituency in Norwich claimed that he had been sent 14 ballot papers — three each for him and his wife at their current address, two each at their previous address, and four for the previous residents of their home. Kevin Kyle, a mortgage adviser, said: “The system is a farce.”

Additional reporting: Abul Taher, Sarah Keenlyside, Chris Whyatt and Holly Watt