This came after David Morris, MP for Morecambe and Lunesdale, wrote to residents asking if they support his campaign to keep Morecambe and Lancaster as separate election seats.
The Boundary Commission for England has proposed combining Lancaster, Morecambe and Heysham for the next General Election.
Cat Smith, the Lancaster MP, told the Commons on Wednesday that letters had not only gone to Morecambe and Lunesdale residents but to people living in Lancaster too.
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Before an MP writes to people in another constituency, it is Parliamentary courtesy to inform the constituency’s MP.
Ms Smith told John Bercow, the Speaker of the Commons, that Mr Morris had not done that.
She said: “Some of my constituents have been left confused, given the subject matter, believing that their MP has already changed under boundary changes.”
But Mr Morris said: “I have not written to the hon. Lady’s constituents by name or used parliamentary paper, resources or a portcullis emblem.
“I also did not deliver any of the letters personally, as I was away on parliamentary business out of the country at the time.
“I have therefore not breached any protocol.”
Ms Smith said afterwards she had been told letters had gone to residents on Abraham Heights and the Marsh estate and she had been expecting an apology from Mr Morris.
Mr Bercow told the Commons: “It is much better if such spats are avoided, and the whole House and all its Members benefit if these courtesies are observed.”