MPs have demanded an overhaul of Westminster’s lobbying system as they criticised David Cameron’s judgment and Treasury obstinacy over the Greensill Capital scandal.

In a stinging 71-page report, the Commons Treasury Committee found the former Tory Prime Minister had not broken any rules when lobbying ministers on behalf of the failed bank.

But it said that was because the rules were too weak and ought to be tougher.

Mr Cameron, who left office in 2016 and became a senior adviser at Greensill in 2018, tried in vain to get state support for the firm as its finances hit the skids in early 2020.

He and his staff sent ministers and officials 73 emails, texts and WhatsApp messages from March 5 until June 26 last year about Greensill. Mr Cameron complained it was “nuts” and “bonkers” that the firm could not a Government-backed coronavirus loan.

Greensill Capital went on to file for insolvency in March this year, threatening 5,000 UK jobs at Liberty Steel, which relied on it for financing.

Steel tycoon Sanjeev Gupta’s GFG Alliance empire owed Greensill around £3.6billion, some of it borrowed against risky “prospective receivables”, money from contracts yet to be signed.

In its Lessons from Greensill Capital report, the Treasury committee said: “We accept that Mr Cameron did not break the rules governing lobbying by former ministers, but that reflects on the insufficient strength of the rules, and there is a strong case for strengthening them.”

It also questioned if Mr Cameron, who held a 1 per cent stake in Greensill potentially worth millions at one point, had been naive about the firm.

It said that if he had “taken a broader and more enquiring assessment of the business” there were “signals available” which might have led him to taking a “more restrained approach”.

The Committee said the Treasury was right to refuse Greensill the loans Mr Cameron was looking for, but said it should have steered him into “more formal methods of communication” when he first got in touch.

The former PM messaged Chancellor Rishi Sunak, Cabinet Office minister Michael Gove, then Health Secretary Matt Hancock, vaccines minister Nadhim Zahawi, and Treasury permanent secretary Sir Tom Scholar, who received 12 texts from him.

MPs were also taken aback by the Treasury’s “firm conviction” that Mr Cameron did not benefit from special treatment as a result of his status.

They said: “We are very surprised about this, given that Mr Cameron was an ex-prime minister, who had worked with those he was lobbying, had access to their mobile phone numbers, and appears to have been able to negotiate who should attend meetings.

“The Treasury’s unwillingness to accept that it could have made any better choices at all in how it engaged in this case is a missed opportunity for reflection.”

Chairman and former Treasury minister Mel Stride said: “Our report sets out important lessons for the Treasury and our financial system resulting from both Greensill Capital’s collapse and David Cameron’s lobbying.

“The Treasury should have encouraged David Cameron into more formal lines of communication as soon as it had identified his personal financial incentives. However, the Treasury took the right decision to reject the objectives of his lobbying, and the committee found that Treasury ministers and officials behaved with complete and absolute integrity.

“We look forward to the conclusions of the other inquiries on the collapse of Greensill Capital, and will continue to follow developments closely.”

Mr Cameron said: “While I am pleased that the report confirms I broke no rules, I very much take on board its wider points. I always acted in good faith, and had no idea until the end of last year that Greensill Capital was in danger of failure.

“However, I have been clear all along that there are lessons to be learnt. As I said to the committee, I accept that communications of this nature should be done in future through only the most formal of channels.

“I agree that the guidance on how former ministers engage with government could be updated and was pleased to provide some suggestions on this to the committee.”

The MPs also said the failure of Greensill Capital, which specialised in supply chain finance, highlighted the risks around new forms of lending, which should be better monitored, potentially with new legislation.

They said the Bank of England, Treasury and Financial Conduct Authority (FCA) should work together to ensure data is more readily released so lenders do not become over-exposed.

In May, Greensill founder Lex Greensill denied being a crook after the FCA launched a probe into “potentially criminal” acts at his collapsed bank.

“No, I’m not,” he said when asked by a Labour MP if he was a “fraudster”.

Several other investigations are taking place into GFG Alliance and Greensill Capital, including ones by the Commons. Business, the Public Accounts and Public Administration and Constitutional Affairs committees.

The Serious Fraud Office is probing GFG and its deals with Greensill.