Ministers will this week face demands to force voters to present photographic identification before they are allowed to vote, as an elections watchdog reveals that police were called in to investigate dozens of allegations of fraud at the last general election.
The Electoral Commission's "analysis of electoral malpractice" will lay bare a catalogue of alleged offences, ranging from postal-vote fraud to false leaflets and bogus voting, at the election in May.
The report, compiled in conjunction with UK police chiefs, follows persistent claims that local polls were being illegally influenced, despite repeated attempts to tighten up the laws to prevent the systematic theft of votes. Critics claim that, in the worst cases, seats were effectively "stolen" from losing candidates.
Details of the alleged offences have provoked calls for a root-and-branch reform of Britain's "19th-century" election laws, requiring voters to present photographic identification – including their passports – before they are allowed to vote.
An Independent on Sunday investigation has established that, during the past eight months, more than half of police forces across the United Kingdom have investigated complaints about the conduct of the election. Police sources say that more than 80 allegations of criminal behaviour, including tampering with ballot papers, were received following the election – and that at least 25 of them resulted in formal complaints and police investigations.
The list of allegations included one in the constituency of the Secretary of State for Justice, Kenneth Clarke, the man who is tasked with ensuring that elections are conducted within the law.
In the months following the poll, the Conservative Party chairman, Baroness Warsi, claimed that fraud had deprived the Tories of at least three seats in the closest general election for almost 30 years.
"At least three seats where we lost, where we didn't gain the seat, [were] based on electoral fraud," she said. "I am saying there are seats at the last election in which those constituencies are concerned, quite rightly, that electoral fraud took place."
Lady Warsi has refused to name the constituencies, although one is believed to have been Halifax, where Labour held on by less than 1,500 votes in May. Tory MP Greg Hands later claimed that 763 postal votes issued in Halifax failed to match voter registration records. It subsequently emerged that two local Tories had been arrested over allegations of electoral fraud in the weeks before polling-day. West Yorkshire Police later dismissed the Conservatives' allegations of irregularities, following a "comprehensive" investigation.
Nottinghamshire Police chiefs confirmed that they had received 10 complaints, including one relating to Mr Clarke's Rushcliffe seat. Ashfield, the former constituency of Geoff Hoon, produced three complaints, and there were two in Bassetlaw, the seat of John Mann, a prominent campaigner for greater transparency in politics.
The complaints included allegations relating to leaflets, "false register", expenses and public order. More than half were made against the candidates themselves.
An allegation of malpractice relating to the details on an election publication in David Cameron's Witney constituency was one of 11 investigated by Thames Valley Police.
The Metropolitan Police launched a number of investigations, including one into allegations of false registration of voters' addresses in Tower Hamlets. Greater Manchester Police received allegations of fraud in a number of areas, including Rochdale, North Manchester, Oldham and Bolton. Deputy Prime Minister Nick Clegg is expected to give a speech on the failings of the voter registration system tomorrow.
But campaigners last night claimed that the continuing complaints – and fears that hundreds more cases of abuse went undetected – meant wide-scale reform was required.
Stuart Wilks-Heeg, the executive director of Democratic Audit, said: "Postal votes are still vulnerable, but we need to change the system of registering voters, which has weaknesses.
"There is also an argument for better security at polling stations, because people can come in with a polling card and no one needs to check who they are."
Peter Facey, of the pressure group Unlock Democracy, said the complaints proved the case for individual voter registration and a requirement for all voters to prove their identity.
He added: "We still have 19th-century regulations for a 21st-century situation. In a lot of these cases we are talking about the possible theft of somebody's vote.
"In Northern Ireland voters have to provide photographic identification; there is no reason why we should not require that in the rest of the UK."