A police force has been accused of threatening the principle of parliamentary free speech by sending a notice to an MP warning him not to harass a constituent.

East Worthing and Shoreham MP 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.

Appearing before the Commons Privileges Committee, Sussex Police chief constable Martin Richards and his former assistant chief constable Robin Merrett apologised for the wording of the notice and for failings in their understanding of parliamentary privilege, but insisted they stood by the decision to issue it. Mr Richards made clear that he had not apologised to the MP himself.

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.

Sussex Police - which had previously investigated allegations of racial hate crime made by Mr Francis against the MP - received a complaint that this mailing caused him distress and alarm, and sent Mr Loughton a PIN, warning that his constituent saw it as part of a campaign of "collective harassment" and that any further behaviour of this kind could leave him liable to prosecution.

Mr Loughton told the committee: "The notion that... the mere sending of the parliamentary record of the House of Commons can constitute an offence of harassment is extraordinary, bizarre and deeply worrying for the way we do our business in this place.

He said that the force's decision to send the notice without taking legal advice or consulting the Commons authorities "strikes me as extreme arrogance, not ignorance" and said it was " completely out of sorts" with Association of Chief Police Officers guidance on the use of PINs.

Mr Loughton said he had nothing to do with a series of anonymous letters which Mr Francis had apparently received. But he said that the PIN's use of the words "collective harassment" appeared to suggest he was "some sort of mafia boss" orchestrating a campaign of abuse.

He warned that MPs' freedom to speak freely and robustly had been "limited by the extraordinary, discretionary, arbitrary and highly subjective actions of Sussex Police, with the absolute endorsement of the chief constable".

He told the cross-party committee of backbench MPs the incident had made him "think twice" before speaking out about issues of concern to his constituents, adding: "I have been put through a huge amount of stress, personal hurt to me and my family and it has seriously impacted on my ability to do the job I was elected to do.

"This case, I think, has very significant implications for the way all of us go about our business in this House and must be challenged, otherwise every time we seek to write a letter to our constituents, every time we seek to raise something in a debate in this House, potentially we will have to seek the permission of the chief constable before we do it, or else risk being subjected to a complaint that could lead to criminal action. I find that deeply chilling."

Mr Merrett, who retired in December, told the committee that PINs were sent to half a dozen people last September in a bid to calm down what had become an "entrenched" dispute between them. Police were also pursuing other ideas, such as mediation between the feuding parties.

"The PIN was an attempt by us to draw a line in the sand regarding the various allegations we had received concerning harassment and give everyone fair notice that people were being genuinely upset by what other people were doing," he said.

Mr Merrett accepted that the police had no evidence linking Mr Loughton to other anonymous letters sent to Mr Francis, but said that he could "understand" how he could have felt alarmed by receiving the copy of Hansard with no indication of who had sent it.

The PIN was not intended to warn the MP that he should not in future send extracts of Hansard to constituents, said Mr Merrett. But he told the committee: "I fully accept and apologise that the wording of the PIN could have been clearer on that point."

He acknowledged that he was "mistakenly confident" about his understanding of parliamentary privilege, and did not realise that the 1840 Parliamentary Papers Act gave full legal protection to MPs sending out copies of reports of proceedings in the House.

"I apologise to the House and this committee for not being aware of the 1840 Act," said Mr Merrett, but added: "I fully support the sending of the PIN."

Mr Richards told the MPs that the force had received legal advice from a QC, after the PIN was issued, that it did not breach parliamentary privilege. But he accepted that there was "conflicting" advice from Commons clerks and said police would welcome guidance.

The PIN was "not a threat, it was information" and was never intended to be seen as a precursor to prosecution, he said.

Mr Richards said he was "extremely sorry" that the force had not been aware of the 1840 Act.

"Our knowledge was not as strong as it should have been in my view and I have apologised for that," he said. "I would say, however, that we are receiving conflicting advices from leading counsel and from the clerk on that issue."

Asked if he had apologised directly to Mr Loughton, Mr Richards replied: "No. I have explained and apologised to this committee."

Committee member Geoffrey Cox, the Conservative MP for Torridge and West Devon, told the chief constable that the police had "blundered" by sending the PIN without properly understanding the law on privilege and warned: "This has a chilling effect potentially on the actions of MPs and it is a very, very serious action.

"Wouldn't you agree that neither you nor your force had a proper appreciation of the gravity of what you were doing by the sending of this notice?"

The Clerk of the Commons Sir Robert Rogers told the committee that the 1840 Act gave complete protection to Mr Loughton.

Sir Robert told the MPs he would have been astonished to think that a single copy of Hansard could result in harassment proceedings, and said MPs would have to be "telepathic" to be able to foresee the reaction of a recipient.