Lib Dem MP John Hemming made a complaint about an e-mail
Parliament's watchdog is to look into claims by an MP that a law firm tried to "intimidate" him into not speaking about its client in the Commons.
Lib Dem John Hemming said Withers LLP had sent him an e-mail saying he should not make statements in the chamber regarding a supermarket development.
Mr Hemming said MPs must be "able to speak on behalf of our citizens".
In a statement, Withers LLP said that the company had "acted entirely properly and professionally".
The Commons has decided that its committee on standards and privileges would look into Mr Hemming's complaint.
Mr Hemming's complaint relates to parliamentary privilege, MPs' right to speak in the House without fear of being gagged or sued for libel.
The Birmingham Yardley MP referred an e-mail, which he received last August from Withers LLP, to Commons Speaker John Bercow, who granted an emergency debate.
In the e-mail, Withers LLP claims its client - who owns land in Yardley and is contesting a compulsory purchase order made in Tesco's favour - has been defamed in a leaflet sent out by Mr Hemming.
The e-mail goes on: "Your threat to make a statement in the House of Commons referring to our client's alleged 'spoiling tactics' in this and other situations and that our client's threatened proceedings amount to 'bullying and an attempt to gag opponents' is tantamount to blackmail.
"These allegations are untrue as our client is only trying to put right a serious wrong to his reputation. We note that you would only make these allegations under the cover of parliamentary privilege.
"My client objects very strongly to you doing this and would ensure, via other sources, that the House of Commons were fully appraised of the true situation and not misled."
In the Commons, Mr Hemming said Withers' LLP's client, businessman Jeremy Knight-Adams, had "wrongly" taken the contents of the leaflet to refer to him.
He added that he had the right to speak out on behalf of elderly residents in sheltered accommodation next to the proposed Tesco supermarket site, whose lives were being affected by living next to unfinished "building works".
Mr Hemming also said: "If the House agrees to this motion it will be up to the Committee on Standards and Privileges to examine not just the wording used by Withers in their email to me of 4 August... but also the full context of the way in which attempts are being made to intimidate a Member of this House and to deter him, by threats ostensibly directed elsewhere, from exercising that freedom of speech enjoyed by this House, as one of the principle foundations of the liberties of the British people."
The government, Conservatives and Liberal Democrats all backed the MP's motion.
Deputy Commons leader Barbara Keeley said: "I would like to give the government's support for the mention of the motion to the committee."
For the Conservatives, shadow deputy Commons leader Shailesh Vara said his party supported the motion "because it impacts on what a member of this House can say".
But Tory MP Andrew Murrison said: "We must be free to discuss things in the House without fear or favour, but that does not relieve us of our responsibility to be extremely careful about what we say outside this place.
"From years of experience, the Liberal Democrats are pretty adept at putting out material of a doubtful nature."
In its statement, Withers LLP said: "Throughout our client's dispute with John Hemming MP, we have acted entirely properly and professionally in defending our client's reputation.
"We strongly refute Mr Hemming's allegations that our actions were intimidatory in any way."
Mr Hemming's motion was passed by MPs without a vote.