A police force committed a contempt of Parliament by sending a notice to a senior MP warning him not to harass a constituent, the Commons Privileges Committee has concluded.
Former minister Tim Loughton said the police information notice (PIN) was issued by the Sussex force after he sent Kieran Francis a copy of Hansard - the record of parliamentary proceedings - containing a speech he made to the House of Commons about a long-running dispute between them.
The committee said Sussex Police "made a serious mistake" and it would be "appropriate for the PIN issued to Mr Loughton to be withdrawn".
The MPs said: "We do not consider further action is necessary in this case, subject to an appropriate response from Sussex Police.
"Nonetheless, we deplore Sussex Police's inability to recognise that the freedom of speech in Parliament, and the ability of Members to carry out their functions without unfounded legal threats, are themselves part of the law which the police uphold."
East Worthing and Shoreham Tory MP Mr Loughton told the committee that after years of abuse, both face-to-face and on the internet, he decided last March to tell the Commons he was "sacking Mr Francis as my constituent".
On the advice of the Clerk of the Commons, he sent a copy of Hansard containing the announcement to his constituent by post, with an unsigned Commons cover slip, as a way of informing him he intended to have no further communication.
The police issued PINs to all those involved in the long-running dispute, but in doing so broke the rules protecting freedom of speech in Parliament, which extends to the publication of Hansard, the committee said.
The report said: "The PIN carries a threat of legal action and it was motivated by the content of Mr Loughton's speech to the House. A threat of legal action arising from the content of a speech made in the House and published by order of the House is clearly a contempt.
"The ability of those who speak in Parliament to speak freely is a fundamental part of our democracy. The prospect that there may be legal proceedings in consequence of the making of or simple publication of such a speech is likely to have a chilling effect on MPs and on others who take part in official proceedings.
"We accept that Sussex Police did not intend to commit a contempt. Indeed they did not consider the matter at all until far too late. While they have refused to consider that their actions in sending the PIN were in themselves inappropriate, they have apologised if they committed a contempt and asked for advice.
"They have clearly made a serious mistake and they have acknowledged as much."
The committee added: "Under the circumstances we consider it would be appropriate for the PIN issued to Mr Loughton to be withdrawn."
The MPs acknowledged that it was a matter for Sussex Police, but said the committee should be informed on whether the PIN had been withdrawn or a further notice issued.
"Sussex Police should inform the committee of the decision that they make and the reason for it," the committee said. "We will not consider the matter closed until they do so."
A Sussex Police spokesman said: "There has been a long-running local dispute involving a number of parties, including Tim Loughton MP.
"We dealt with this as an operational matter, as we would any other, and in issuing PIN notices to all parties, our aim has always been to reach a resolution, treating everyone fairly and in accordance with the law.
"Police information notices are used to make someone aware that their actions have prompted an allegation of harassment. They do not in themselves constitute any kind or threat of formal legal action and are issued with the intent of preventing further escalation of an issue.
"The question of parliamentary privilege and associated legislation is highly complex, as was evidenced by the debate that took place at the committee of privileges.
"The committee recognises that our breach was inadvertent. We will give the report and any recommended action careful and serious consideration."
Sussex Police and Crime Commissioner Katy Bourne said: "I welcome the inquiry by the Parliamentary Privileges Committee and I will expect Sussex Police to give very careful consideration to the recommendations outlined within their report.
"Following the oral evidence given by Sussex Police to the committee, I felt that there was an immediate need to understand how effective the existing national guidance is on the issuing of Police Information Notices (PINs) and whether the manner in which Sussex Police is issuing PINs is appropriate.
"As a result I have already asked the College of Policing to undertake a review of these issues. As police and crime commissioner I will ensure that Sussex Police fully embraces any learning from the review and works closely with the College of Policing to formulate improved policy in this area."