Senior Cabinet minister Jacob Rees-Mogg is under investigation by the Commons standards watchdog, just weeks after attempting to rip up the rulebook on scrutinising MPs' behaviour.
The office of Parliamentary Commissioner for Standards Kathryn Stone said the Scottish Tory leader Douglas Ross would also be investigated, both over allegations that they failed to register interests.
It is understood the probe into Mr Rees-Mogg will investigate claims he failed to register with the Commons £6 million in personal loans from a company linked to the Cayman Islands for which he was previously a director, following a report by the Daily Mail.
He borrowed millions in the form of "director's loans" from Saliston LTD between 2018 and 2020, according to the paper.
MPs are required under their code of conduct to declare "taxable expenses, allowances and benefits".
Mr Ross, who is a Westminster MP as well as a member of the Scottish Parliament, is under investigation in relation to his financial interests.
Last month he referred himself to the Standards Commission after he was revealed to be among a number of MPs receiving payments for second jobs, with Mr Ross also employed as a football referee, for which he allegedly failed to register his earnings.
Speaking to the Scottish newspaper the Herald, Mr Ross said: "Since realising my mistake last week, I contacted the Office of the Register of Interests and made them aware of the situation.
"All payments have now been declared, including those from my MSP salary that are donated to charities."
There's an increased focus on Mr Rees-Mogg, however, given his role in attempting to change how the Standards Commission operates.
In mid-November he admitted making "a mistake" by urging the prime minister to change the standards process in order to give his friend Owen Paterson the chance to appeal a punishment recommended by the Standards Commission for breaking lobbying rules.
The government was eventually forced into an embarrassing U-turn and Mr Paterson eventually resigned and numerous allegations against other Tories followed.
Labour wants Boris Johnson to launch an investigation into Mr Rees-Mogg, to see whether he broke the ministerial code.
The party is calling on Mr Johnson to ask Independent Adviser on Ministers’ Interests Lord Gedit to investigate, with deputy leader Angela Rayner saying "ministers act like they are above the rules because they think that there is one rule for them and another for the rest of us".
Mr Rees-Mogg is understood to be fully complying with the standards commissioner's investigation.
Defending himself to the Daily Mail at the time of the original report, Mr Rees-Mogg said the company in question, Saliston, is "100 per cent owned by me. 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 from 2018 were primarily taken out for the purchase and refurbishment of [my 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. "
A statement from Mr Rees-Mogg, following the launch of the investigation, 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."