The Government has "nothing to fear" from an investigation if David Cameron's alleged lobbying of ministers was "above board", Labour has said.
Mr Cameron is under investigation by the lobbying watchdog over claims he tried to persuade the Chancellor to grant loans to a financial firm he worked for.
The ex-PM is reported to have sent a number of text messages to Rishi Sunak ’s private phone asking for support for Greensill Capital through the Government's Covid Corporate Financing Facility (CCFF).
He is also alleged to have approached the Bank of England about the firm, which collapsed into administration earlier this month.
The Transparency of Lobbying, Non-Party Campaigning and Trade Union Administration Act 2014 makes it an offence for someone who is not a registered lobbyist to directly lobby ministers or senior civil servants.
The offence could result in a fine of up to £7,500. But people lobbying on behalf of their own organisation are not required to register and it is understood that Mr Cameron was an employee at Greensill.
His activities are now being investigated by Harry Rich, the Registrar of Consultant Lobbyists - a post set up in legislation passed by Mr Cameron's Government in 2014.
In 2010, Mr Cameron warned lobbying in Westminster would be “the next big scandal waiting to happen.”
Opposition MPs turned their guns on the former PM as they debated the future of the steel industry, with Greensill the main lender to Liberty Steel.
Asked about the investigation following a speech in London, Shadow Business Secretary Ed Miliband said: “I think there does need to be an investigation. I think it's the right thing to do.
“I think if I were in government and it was about someone as senior as a former Prime Minister I would want an investigation to take place. If indeed it was all above board and fine, then the government should have nothing to fear from having an investigation."
Labour MP Rupa Huq told the Commons: "The Greensill affair raises not just Liberty Steel's refinancing, but David Cameron, ex-PM, bending the ear of the now Chancellor, although he wasn't on the lobbying register.”
Raising fears about “transparency and conflicts of interests”, she added: “This Government seems to operate a culture of friends with benefits and mates' rates, with jobs in British steel just being mere collateral."
Lashing out at “unsolicited advice from a former Prime Minister”, SNP MP Dave Doogan said: “We know David Cameron used his direct line to contact the Chancellor regarding Greensill's difficulties.”
Labour's Geraint Davies said: "We know that David Cameron was an adviser to Greensill Capital with shareholdings of potentially tens of millions of pounds and that he made private texts and calls on a number of occasions to the Chancellor to help secure funds for Liberty before Grensill, a high-risk company, went bust, which has put thousands of jobs at Liberty Steel at risk.”
Calling for an investigation by the Business Department, he urged Business Secretary Kwasi Kwarteng to “ensure in future that taxpayers' money no longer is interfered with by David Cameron and former Conservative ministers but instead invested directly to protect our jobs in British steel and other vital industries”.
Labour former Treasury Minister Dame Angela Eagle branded Greensill “an unregulated shadow bank with close links to the Conservative Party”.
Mr Kwarteng said: “Officials often meet with a huge number of businesses, business people, who are affected by policy, that's part of the policy development.
"But it's done in a transparent, open and proper way always."
