City firm Withers faces an investigation into claims that it committed a contempt of parliament by seeking to prevent an MP from talking about one of its clients there.

The Commons held an emergency debate on the matter last Thursday, and it was referred to the Committee on Standards and Privileges which is currently considering what to do.

The allegation concerns an email sent by Withers to John Hemming, Liberal Democrat MP for Birmingham Yardley. Hemming claims the email sought to ‘intimidate’ him in his parliamentary conduct.

In the message, Withers claimed that a leaflet distributed by Hemming was defamatory of its client, Jeremy Knight Adams, who is contesting a compulsory purchase order granted to supermarket Tesco. The client was not named in the leaflet. Withers sought an undertaking that Hemming refrain from repeating the remarks.

The email contained the passage: ‘In order to settle this matter we, therefore, require an apology in respect of both the serious allegations plus payment of our client’s costs, a substantial payment to a charity of his choice and an undertaking not to repeat the allegations or any similar allegations, particularly in parliament.’

The leaflet related to a long-running planning dispute over a supermarket development. Hemming had become involved when he took up the concerns of residents living in sheltered flats near the development site. He told the House of Commons that the leaflet did not relate to Knight Adams.

A statement from Withers said: ‘We believe that we have acted entirely properly and professionally in asserting our client’s rights and are confident that this will be proved to the committee.

‘In defending our client’s ­reputation, at no stage did we attempt to limit Mr Hemming’s parliamentary privilege… In attempting to settle our client’s defamation case, we acted in accordance with ­standard legal practice – namely seeking an undertaking that Mr Hemming would not repeat the defamatory remarks about which our client complained.’

It continued: ‘Our email of 4 August 2009 was sent in this context. Subsequent to this email, an unreserved apology was issued by Mr Hemming.

‘The reference in the email to parliament was only made in direct response to an assertion by Mr Hemming that he would "make reference to [the allegations] at my earliest opportunity in a speech in the House of Commons". Our email simply outlined that if Mr Hemming was not prepared to give an undertaking that he would not repeat the defamatory allegations, then our client would not be able to settle his dispute.’