A Liberal Democrat candidate in the 10 June local elections in Oldham has been arrested over allegations of attempted voting fraud.
The arrest comes in one of the all-postal vote trials areas
Aftab Hussain, 39, was arrested on Wednesday on suspicion of conspiracy to defraud and theft of ballot papers.
It comes after two men allegedly called at a house in Glodwick on Friday and offered to look after ballot papers for residents - they left with five papers.
The family contacted police the following day to raise concerns.
Liberal Democrat leader in Oldham Howard Sykes confirmed Mr Hussain had been arrested. Mr Hussain is due to answer bail at Oldham police station on 5 August.
A spokesman for the party said: "We do not condone any illegal activity in relation to postal votes, but we fear this system is open to abuse.
"The abolition of polling stations means that people cannot be guaranteed the right to vote in privacy and security.
"All the problems that have arisen with the system of postal voting should be carefully reviewed after the elections."
Some MPs and experts have raised fraud concerns about postal votes, but ministers and the elections watchdog insist the system is no more vulnerable than conventional ballots.
Chief Inspector Stuart Harman, from Oldham Police, said the force took election irregularities very seriously.
"This is the first time a postal vote is being piloted across the North West and in order for it to work, voters must have confidence in the system," he said.
"Although a number of precautions have been put in place to prevent any type of fraud at this year's elections, we are relying on members of the public to report any unusual circumstances to their local police."
Sir Howard Bernstein, returning officer for the North West region in the local and European elections, said he was pleased people were being vigilant and that the police were investigating malpractice claims.
"Over the last few months the local councils, the police, the government and all our other partners have worked really hard to develop procedures and safety precautions against those who try to abuse the system.
"This arrest is further evidence of how seriously we will act in relation to election offences."
Allegations of irregularities have tarnished previous local elections in Oldham.
In April 2002, detectives investigated alleged
vote-rigging ahead of the local elections. It had been claimed some households had an excessive number of voters registered.
In 2001, 10 people were convicted of election fraud over the council elections a year earlier.