The justice minister, Jonathan Djanogly, failed to declare a third company owned by his brother-in-law, which supplied staff to an industry he regulates, the Guardian can reveal.

Djanogly, one of the richest MPs and heir to a £300m fortune, recorded in the ministerial register that his brother-in-law, Ben Silk, "owns Going Legal Ltd and Legal Link Introductory Services Ltd".

Both are part of the claims management industry, consisting of companies which chase the sick, the sexually harassed and the sacked and put them in touch with no-win no-fee lawyers for cash.

But the minister did not declare that another firm run by Silk, Matrix UK, also provided staff for the claims management companies: it supplied Djanogly's niece to work for Going Legal.

It was announced on Monday that Djanogly would step down from regulating the claims management industry, after an investigation by Sir Gus O'Donnell, the cabinet secretary – a move which led the Law Society, the largest legal professional body, to call for Djanogly's legal aid bill to be "paused" in parliament.

The Cabinet Office investigation was sparked by Guardian revelations that the minister and his family could profit from the controversial changes to legal aid he was piloting in parliament.

In an interview with the Guardian, Silk confirmed that staff were "employed by Matrix ... but working for Going Legal" responding that with his own daughter this was the "technical position, yes".

All three firms – Matrix, Going Legal and Legal Link – operate from the same central Manchester address and share the same switchboard. They are majority-owned by Silk, who is a director of all three firms.

Djanogly had last month been forced to sell his teenage children's stakes in Silk's companies because he was regulating them. Silk said this was an honest mistake. "I had never discussed my gift of the [children's] shares with Jonathan," said Silk.

Last night a spokesman for the minister said: "Jonathan Djanogly has no interest, financial or otherwise, in Matrix UK and has no knowledge of the company's activities." The minister is expected to return to parliament next week to push through the legal aid changes.

The ministerial code states: "Ministers must provide a full list in writing of all interests which might be thought to give rise to a conflict. The list should also cover interests of the minister's spouse or partner and close family which might be thought to give rise to a conflict."

Labour said the ministerial code made it clear that any financial interest should be declared. "The minister has complied with two companies but not with this holding company the Guardian has discovered." The shadow justice minister, Andy Slaughter, said he would be writing to the cabinet secretary with a "range of questions about the minister's possible conflict of interests and the consequences of his supervision of legislation".

Djanogly married Silk's sister in 1991. Silk has a long career in insurance broking and claims management - his first insurance company Growth Alliance was set up in 1988 and went bust in 1991.

The 51-year-old was discharged from his bankruptcy in 1994. Upon returning to business Matrix bought into two insurance companies in 1998, although both went under after just 24 months.

Silk said that Matrix was "probably" not registered by Djanogly because its main activities "did not involve the insurance sector or the legal sector". He said: "It's a consultancy and if someone wants to know how to sell grapes and comes to me then Matrix will do the work."

Last year Matrix posted a profit of almost £64,000. Silk's other two firms made a profit of £130,000 last year. Both are successful businesses. Legal Link Introductory Services, which trades as Justice Direct and markets itself as the "leading claims management company in the UK",

The other is Going Legal, which claimed it has recovered £20m for unfair dismissal claims since 1999. It is also known as "Sexual Harassment at Work", "Employment Tribunal Claims" and "Injury At Work Claims".