The UK financial regulator is “formally investigating” Greensill Capital as documents released by a parliamentary committee showed former prime minister David Cameron lobbied for the company 56 separate times last spring.

The Financial Conduct Authority disclosed the probe into the collapsed supply-chain finance group in a letter from Nikhil Rathi, the FCA chief executive, to Mel Stride, the Conservative MP who chairs the House of Commons Treasury select committee.

Greensill went into administration in March. Rathi said the FCA was co-operating with authorities in Germany, Australia, Switzerland and other countries.

The letter to Stride emerged in a release of documents by the committee. The release included a deluge of friendly messages from Cameron — who was an adviser to Greensill — to government and Whitehall figures in early 2020 imploring them to help the company.

The documents showed how Cameron bombarded cabinet ministers and officials, including Cabinet Office minister Michael Gove, via text, WhatsApp, email and phone calls in an attempt to change the rules around Covid-19 debt schemes to the benefit of Greensill Capital.

He wanted supply-chain companies to gain access to the Bank of England’s Covid-19 debt scheme. Although that attempt failed, Greensill did go on to access £400m through another government loan scheme for its clients.

In a message to chancellor Rishi Sunak on April 3 2020, Cameron praised the chancellor: “You are doing a great job — keep going.”

The former prime minister also messaged Sir Tom Scholar, the permanent secretary at the Treasury. “See you with Rishi’s for an elbow bump or foot tap. Love Dc,” [sic] he said.

At one point he apologised to Scholar for hassling him: “One last point then I promise I will stop annoying you.” The next day, left empty-handed, he said: “Am now calling CX (Sunak), Gove, everyone.”

Lex Greensill, the company’s Australian founder, told a hearing of the Treasury committee on Monday that the withdrawal of insurance was the “ultimate” cause of the group’s demise.

The collapse of Greensill has prompted concerns about the finances of Sanjeev Gupta’s GFG, a major Greensill client which employs thousands of Liberty Steel workers in the UK.

In three hours of robust questioning by MPs, Greensill denied that he was a “fraudster” given the company’s heavy use of future receivables — lending based on revenue which does not yet exist. “At no point would I or my firm have engaged in financing receivables which we knew to be fraudulent.”

Asked what proportion of his company’s lending went to Gupta-related businesses, Greensill refused to comment on “specific clients”, citing legal advice. He said GFG was not Greensill’s biggest client but admitted: “We did have a concentration on certain customers that was too high.”

Greensill was asked how he had been brought into Whitehall in 2012 to advise the Cameron government on supply-chain finance. “I was simply trying to share my experience and give something back,” he said.

Explaining how Cameron then came to work for him in 2018, the Australian financier said the former prime minister had been a “PAYE employee” and not a director, with “less than 1 per cent” of share options in the group. At one point Greensill had a valuation of about $7bn, making Cameron’s stake worth up to $70m.

Cameron had a “standing invitation” to Greensill board meetings and regularly attended, he said.

Greensill added that the company bought four private jets because it was “an efficient way of getting around”.

The Treasury told the committee of MPs in a separate submission that it had conducted a detailed assessment of Greensill’s proposal and concluded it did not provide value for money.

Cameron is scheduled to appear before the Treasury select committee on Thursday.