Theresa May’s chief spin doctor formalised her resignation from three companies set up with a public relations executive days after Labour raised questions of a potential conflict of interest.

Katie Perrior became the prime minister’s head of communications in July last year. She notified Companies House on 21 December that she had stepped down as a director of the public relations companies iNHouse PR, iNHouse Connex and Hersay Ltd.

Perrior, whose role was occupied by Alastair Campbell and Andy Coulson under previous prime ministers, was the co-director and shareholder in all three firms with Jo Tanner, a former Conservative party staffer and long-term business partner.

In a statement to the Guardian, Perrior, the former head of press at ITV and Channel 4 News, blamed an administrative oversight for the failure to register the resignations when she took up the Downing Street role in July. Companies House documents show she has now backdated her resignations to that date. She stood down from from the companies in July, but this was not properly filed at Companies House by her accountants, she added.

The Labour MP for Grimsby, Melanie Onn, asked the civil service’s head of ethics, Sue Gray, on 19 December why the three companies had not been dissolved and the shares disposed of in July.

Questions were also raised about Perrior’s working relationship with iNHouse Communications, a fourth company from which Perrior resigned in July, where Tanner remains a director.

Responding after the resignations, Onn said: “Katie Perrior resigned three PR company directorships and ditched shares but this has been registered just two days after I raised them with the civil service’s head of ethics and propriety.

“It should not have taken my intervention for this potential conflict of interest to be identified,” she said.

Onn questioned why the Cabinet Office had not replied to her letter.

“[Downing Street] should be keen to clarify what rules they put in place around the outside business interests of Theresa May’s top spin doctor,” she said.

Perrior stood down from iNHouse Communications to work on May’s leadership campaign and was then recruited to take up the crucial role in Downing Street. The agency, set up with Tanner, ran Boris Johnson’s PR during his 2008 bid to become London mayor, and his successful 2012 re-election campaign.

Its extensive client list has included Santander and the Jamie Oliver Foundation. Since joining Downing Street, Perrior has appeared at some of the agency’s official events.

Partygoers said she was present at a karaoke night at this year’s Conservative party conference, held the day before Theresa May’s speech to the Conservative faithful and attended by Number 10 staff and journalists. She has also attended a press event for lobby journalists held to promote the West End play This House, which is set in the corridors of Westminster. iNHouse Communications is promoting the play.

At Labour and Conservative party conferences, the agency organises the London Lounge, an exclusive meeting space sponsored by Starbucks, Rupert Murdoch’s News UK and the drinks giant Diageo.

Ministers, special advisers and shadow ministers use the space during the day to hold meetings with lobbyists and journalists. Access to the space is limited on the door and iNHouse staff co-ordinate table space for ministers.

Meetings with lobbyists have to be declared by ministers or special advisers if organised through official channels in Whitehall, but meetings at party political conferences do not.

At night, the London Lounge is used to host drinks events with ministers, including functions for the Times, the Sunday Times, the Sun and the Sun on Sunday.

Company law stipulates that directors should inform Companies House within 15 days when they resign. Companies House says it rarely pursues directors for failing to declare a resignation on time and usually accepts the word of a director if asked to backdate a resignation.

A spokesperson for the Cabinet Office said: “Ms Perrior informed the Cabinet Office that due to an administrative oversight, her accountants had not registered her resignation on the documents relating to the dormant companies, only the ones that were trading.

“This has been rectified and is shown on the notes at Companies House which clearly state that Ms Perrior resigned from all companies on the 14 July 2016.

“On appointment as a special adviser, Ms Perrior informed the Cabinet Office of her business interests. She confirmed she had resigned from the board of iNHouse Communications and that she had given up her shareholding in the company. She has no continuing relationship with this company.”

• The headlines on this article were amended on 23 February 2017. A line was also added to the article to clarify that Perrior stood down from the three companies in July, but said the resignations were not properly filed at Companies House by her accountants.