Douglas Ross welcomed Downing Street's announcement of a review into how the collapsed financial firm Greensill Capital was given Government support amid lobbying by former prime minister David Cameron.
Senior lawyer Nigel Boardman will examine how the specialist bank – founded by Australian financier Lex Greensill – was granted access to a Covid loan scheme for businesses, putting hundreds of millions of pounds of taxpayers' money at risk.
The firm later collapsed into administration, but not before Mr Cameron unsuccessfully lobbied ministers on its behalf in a bid to ask for support for Greensill through the Government's Covid Corporate Financing Facility
A Freedom of Information request revealed text messages sent by current Chancellor Rishi Sunak to Mr Cameron, including one that said he had "pushed" the Treasury to look at what support it could provide during the coronavirus pandemic.
Questions have since been raised about whether Mr Sunak's actions, including a planned phone call between the men, may have broken the ministerial code.
Asked about Mr Sunak's actions, Mr Ross said the Government's review would look at a "broad range of issues" that have emerged and he insisted any breach of the code would be an "extremely serious issue".
The Conservative MP, who resigned as a UK Government minister over its defence of Dominic Cummings's journey to Durham and Barnard Castle during the first lockdown, said he hopes the inquiry will look at all the allegations being made.
A recent investigation into bullying by home secretary Priti Patel was overruled by Boris Johnson and she remains in post, prompting the Prime Minister's former standards adviser Sir Alex Alan to quit.
Commenting on the Greensill lobbying affair, Mr Ross said: "I think it was absolutely right that the Prime Minister has instructed an independent inquiry into this and he has said that that inquiry will be able to look at all aspects where there are currently concerns.
"I think people do raise serious, legitimate concerns on this issue and that's why it's great that an independent inquiry looks into it, looks into all the evidence and reports back."
Mr Ross had called for Nicola Sturgeon's resignation when he believed the First Minister had broken the code for Scottish ministers, and again when a Holyrood committee concluded she had misled Parliament over her meetings with Alex Salmond.
Asked if he would call for the resignation of Mr Sunak – or any other minister – if they had not followed the rules for ministers' behaviour, Mr Ross said: "Of course we have to look at what the inquiry comes back with, but if someone has breached the ministerial code that is an extremely serious issue, and they would have to consider their position, absolutely."
Challenged previously about whether he would hold Conservative and SNP ministers to the same standards, Mr Ross told PA: "I will always do things impartially on the evidence that comes to light."