A Shropshire MP has said he has been told that Labour Party councillors who work at his local sorting office are "manipulating postal votes".
Daniel Kawczynski's comments have parliamentary privilege
Daniel Kawczynski, Conservative MP for Shrewsbury and Atcham, told the Commons he was concerned by the allegations but had not contacted police.
He also said Labour's parliamentary candidate John Tandy was "implicated in various fraudulent practices".
Mr Tandy denied the allegations and called for the MP to resign.
Speaking during a debate on the electoral system, Mr Kawczynski said he had been told "allegations" about Labour members who worked at Shrewsbury's sorting office.
He said: "One ultimately thinks, 'Yes, they work in the post office. They could, as Labour party activists, play some role in manipulating postal votes'."
He went on to speak about Mr Tandy, a Shropshire county councillor who has been selected as Labour's parliamentary prospective candidate.
"Now I find, as I said, that the Labour party has selected somebody who works at Shrewsbury sorting office to stand against me.
"I am starting to get allegations that he, when he stood to be a councillor, was also implicated in various fraudulent practices."
MPs cannot be sued for libel for any statement they make in Parliament. There was some disquiet in the Commons that such allegations were being made.
Mr Tandy denied Mr Kawczynski's allegations and called for the MP to resign.
"I am asking Daniel to take those to the police and if these allegations are found to be untrue, which they are, then I am asking him to think of his position as an MP.
"Making allegations against a member of this constituency, against a person who is just trying to do his job - I ask him to resign his position as MP of Shrewsbury and let's have an election in Shrewsbury to find someone who doesn't make allegations against people who are only just trying to do a job of work."
Senior Liberal Democrat MP, Simon Hughes, spoke in the Commons immediately after Mr Kawczynski.
He said: "I think that his comments are inappropriate.
"The only way to deal with such matters is to do as I have always done, which is to use the Post Office complaints procedure."
Ian Riley, regional director for the Labour Party in the West Midlands, said: "If Mr Kawczynski has any evidence then he should refer this matter to the police.
"And when the police find that the allegations are unfounded then we will be asking Mr Kawczynski to consider his position as a Member of Parliament.
"I'm sure that the people in Shrewsbury do not want an MP who makes unfounded and cowardly allegations, let alone one who is only prepared to make them in the House of Commons where he can hide behind parliamentary privilege."
Royal Mail spokesman Mike Dalton said: "Royal Mail has policies in place to ensure our people do not allow outside interests - whether political or other - to interfere with their duties."