David Cameron and Rishi Sunak could be hauled before MPs to give evidence on the lobbying row engulfing Westminster.
MPs on the Commons Treasury Committee will call both Tories to give evidence to their inquiry into the fallout from the collapse of the firm Greensill Capital.
Mr Cameron has come under sustained criticism after it emerged that he privately texted the Chancellor on behalf of the firm, where he was hired as a paid adviser after leaving No10.
The Treasury released Mr Sunak’s texts, showing that he told the ex-PM he would “push the team” to see if there were ways the firm could access emergency Covid support.
But Greensill’s request was ultimately refused and the firm filed for insolvency in March, putting thousands of British steel jobs at risk.
The powerful Treasury Committee also wants to question the firm’s founder Lex Greensill, who was an unpaid adviser in Downing Street during the coalition years.
The Bank of England, Financial Conduct Authority and UK Government Investments will also face questions in the probe into the failure of Greensill and its attempts to lobby the Government.
Treasury Committee chairman Mel Stride said: “There are questions to be answered in relation to Greensill Capital regarding the operation of the UK’s financial system and its regulation.
“Also, whether the Treasury responded appropriately to lobbying from Greensill during the pandemic.
“I will be writing shortly to the Chancellor, the Governor of the Bank of England, chief executive of the FCA, and David Cameron requesting answers to various initial questions.”
Mr Cameron, Mr Sunak and Mr Greensill have all indicated that they will cooperate with inquiries into the row.
Mr Cameron admitted that he should have used only the “most formal of channels” to contact ministers but said he had not broken any rules.
Seven inquiries have been launched into the collapse of Greensill Capital and the fallout from Mr Cameron’s lobbying activities on behalf of the firm.
The Commons Public Administration and Constitutional Affairs Committee will look at lobbying rules and whether the penalties are tough enough.
It said the Greensill scandal had “raised significant concerns about the propriety of governance in this country” which “risks undermining public trust”.
Committee chairman William Wragg MP said: “Standards in public life are vitally important, and I think most politicians and civil servants fulfil their roles honourably.
“Maintaining and defending that honourable position matters – that’s precisely why this affair has to be scrutinised.
“We will look into whether the rules need tightening up and clarifying and we will make any necessary recommendations without fear or favour.”
The MPs will examine whether codes of conduct for ministers, special advisers and officials are effective, how conflicts of interest are managed and whether the business appointment rules are broad enough.
They will also scrutinise whether lobbying should be regulated and consider the issues around the use of consultants and contractors in government.
Boris Johnson sought to play down the links between Whitehall and the private sector, insisting there are not “loads of people” working as civil servants while also employed by a business.
The Prime Minister has asked lawyer Nigel Boardman to investigate after it emerged that former government procurement chief Bill Crothers worked as an adviser for Greensill Capital while in his Whitehall job.
The Prime Minister told reporters on a campaign visit to Gloucestershire: “I just want to stress one thing to people who are sort of vaguely tuning in to this.
“People should not, in my view, form the impression that the upper echelons of the British Civil Service have got loads of people who are double-hatting, as it were, doing two jobs – it just isn’t true.
“We’ve got one of the best civil services in the world.
“They are fantastically hard-working people, they have been doing an amazing job throughout this Covid pandemic, apart from anything else, and I just wouldn’t want people to get that impression.
“It is simply not the case.”
Cabinet Secretary Simon Case last week ordered Whitehall department chiefs to declare any instances of senior officials performing dual roles outside the Civil Service.