The MP was asked to listen to the concerns of Cornish residents
A Conservative MP has been criticised for his behaviour as shadow minister for Cornwall given that there is no equivalent government post.
Mark Prisk, MP for Hertford and Stortford, was accused of breaking parliamentary convention by undertaking case work outside his own constituency.
Lib Dem MP Matthew Taylor queried the practice of shadowing a "fictional" minister but Mr Prisk did not comment.
The Speaker said it was wrong for MPs to "interfere" in other constituencies.
Mr Taylor, MP for Truro and St Austell, raised the issue in a point of order in the Commons.
He said the Conservatives appeared to be trying to "get round" a convention that MPs do not involve themselves in constituency business outside their own backyard by appointing shadow ministers for groups of constituencies they did not represent.
He said he had "numerous examples" of Mr Prisk - appointed to the role in 2007 - involving himself in constituency business including "presuming" to act for a local arts organisation which he had also represented.
"In every case this activity as shadow minister of the imaginary minister for Cornwall mirrors the work of local MPs and is clearly for party political purposes using the membership of this place to raise constituency issues as, in effect, a shadow local member of Parliament," he said.
"Is this acceptable under the conventions governing us and if so should all the parties now feel free to appoint MPs as shadow ministers for constituencies to act for party political advantage on local issues over the heads of the local MP?"
I do not expect any member of this House to take up cases other than that in their own constituency. It is wrong
Mr Prisk, who was elected to Parliament in 2001, was named shadow minister for Cornwall in 2007.
At the time, Conservative leader David Cameron said Mr Prisk was ideal for the role as he had been educated in Truro and knew the county well.
In a broadcast on the YouTube video sharing website, Mr Cameron said the appointment was a "great step" in ensuring the party had "policies which address the needs and aspirations of people who live in Cornwall".
Labour had "completely ignored" the county, Mr Cameron claimed, and the Conservatives would not do the same.
He added: "What I want is him to listen to people in Cornwall, to understand their concerns, get to grips with the issues and to report back to me so that Cornwall is heard at the heart of the shadow cabinet and the heart of the Conservative Party so we can address the concerns people have."
The Conservatives do not have any MPs in Cornwall and are hoping to make a breakthrough at the next election.
Responding to Mr Taylor, Speaker Michael Martin said it was "clear, common sense" that MPs should not interfere in other constituencies.
"I do not expect any member of this House to take up cases other than that in their own constituency," he said. "It is wrong.
"MPs guard jealously the fact that their constituency boundary and all those within that constituency boundary are there to be looked after by each individual MP."
Mr Martin, who represents Glasgow Springburn, said he was not "telling off" Mr Prisk, who chose not to respond to the claims in Parliament and could not be reached for comment.
Mr Martin said he would not "like it if someone was taking cases up in my constituency" but deemed the matter closed.