Commons leader Jacob Rees-Mogg is being investigated over a possible breach of rules on declaring outside earnings.
Commons standards commissioner Kathryn Stone did not reveal details of the allegations.
But they are understood to relate to £6m in loans Mr Rees-Mogg received from one of his own companies to help buy and renovate his Westminster home.
Mr Rees-Mogg borrowed up to £2.94m a year in director’s loans from Saliston Ltd, a UK-based property company owned by him, between 2018 and 2020.
Mr Rees-Mogg said said the loans had been properly declared, after the story was first reported in the Mail on Sunday.
In a statement, he told BBC News: “Saliston is 100% owned by me and this is declared clearly in the Commons register and to the Cabinet Office. It has no activities that interact with government policy.”
He said the loans were primarily taken out for the purchase and refurbishment of his Westminster home “as temporary cash flow measures”.
“All loans have either been repaid with interest in accordance with HMRC rules or paid as dividends and taxed accordingly,” he added.
“The register asks for earnings, not loans, which is why I was declared an as a non-remunerated director until I resigned on entering government. Loans are not earnings and are not declarable in the register of interests.”
Mr Rees-Mogg resigned as a director of Saliston in 2019, but his wife Helena remains on the board.
A spokesman for the North East Somerset MP said he would comply with commissioner’s investigation fully and answer all questions.
It might well be legal but the “arrangement” stinks of self aggrandizement …