A Labour MP has been told to apologise for writing a letter to constituents ahead of the 2019 election containing a "personal campaign message" on Brexit.
Dr Rosina Allin-Khan broke the rules over the use of Commons stationery, and for using constituents' contact details without consent, the standards commissioner found.
The commissioner said it was the third time Dr Allin-Khan had broken the rules in three years.
She must reimburse the full costs.
Under the Commons rules, MPs can only use parliamentary stationery for their work as an MP, and are banned from using it for campaigning.
Parliament's Standards Commissioner Kathryn Stone conducted an inquiry following a complaint from a constituent that Dr Allin-Khan had misused stationery, and their contact details, which they had previously provided confidentially.
On 2 November 2019, Dr Allin-Khan sent 1,624 letters - entitled Brexit Update - three days before Parliament was dissolved ahead of the General Election.
Ms Stone said that the letter - that was written on House of Commons headed paper and used pre-paid Commons envelopes - was delivered during a period of "heightened sensitivity".
The commissioner concluded that the letter used "public resources to highlight to a group of voters her position and record on an issue that was one of the key themes of the imminent election" and "contained an important personal campaign message, which...had the effect of seeking to support Dr Allin-Khan's return to office".
Dr Allin-Khan, who is Labour's shadow cabinet minister for mental health, was also found to have broken the rules about the use of personal data, after it was found that the complainant had received emails and a hard copy of a Labour party newsletter, having previously provided his contact details in an email to her about the General Election.
The commissioner concluded that the complainant "was entitled to expect that those details would be held securely and would not be used for another purpose without his consent".
In its report on the commissioner's findings, the Commons Committee on Standards noted that "this is the third time Dr Allin-Khan has been found to have breached the rules in three years; and that Dr Allin-Khan is an experienced Member of the House of Commons, and a frontbench spokesperson."
The committee recommended Dr Allin-Khan should reimburse the full £1,142.52 cost for the improper use of the stationery, and apologise to the House of Commons in a personal statement.
The committee also added, "any further breach of the Code of Conduct by Dr Allin-Khan, where there is evidence of a lack of attention to the rules of the House, might call for a more serious sanction."
A spokesman for Dr Allin-Khan said she apologised for the breaching the rules and "fully accepts the committee's recommendations".
He added that she had "already offered to reimburse the costs", and that steps had been taken to "ensure this does not happen again".