Boris Johnson was today warned of the dangers of “government by WhatsApp” as new revelations emerged about “persistent” lobbying by David Cameron and the failed finance company Greensill Capital.

Fresh disclosures revealed how Mr Cameron made a barrage of increasingly desperate appeals to Treasury officials and the Bank of England for support for the ailing company as sources of income dried up in the early weeks of the Covid-19 pandemic.

The Treasury’s top civil servant confirmed that he took a call from Cameron on his mobile because he had previously worked for him during his time as prime minister, telling a parliamentary inquiry that it was “quite natural” for Whitehall officials to accept approaches from ex-ministers and private sector individuals who they knew.

Meanwhile, Mr Johnson launched an internal Cabinet Office inquiry into the leak of his text message exchanges with businessman James Dyson, in which he offered to help “fix” the vacuum cleaner tycoon’s tax problems.

The move represented a U-turn less than 24 hours after Downing Street indicated there would be no probe. But the inquiry will not cover the leak of messages from Saudi Crown Prince Mohammed bin Salman asking the PM to intervene in a planned takeover of Newcastle United FC, and its report will not be made public.

The prime minister’s official spokesman did not deny reports that cabinet secretary Simon Case urged Mr Johnson to change the mobile phone number which he has had for over a decade because of concerns at the number of contacts he was receiving from people outside government, though a senior Whitehall source insisted the claims were not accurate.

Labour called on the government to release details of every call, text and meeting with Mr Cameron.

Leader Sir Keir Starmer said: “Many members of the public – and I think particularly about all those businesses that have struggled over the last 14 months – would say, ‘I would have loved to have had the prime minister’s number to text him to sort out my tax problem to help my company’. 

“This is about those that can access the prime minister by texting him and everybody else who’s left out.”

As the first of a series of parliamentary inquiries began into the Greensill affair, Treasury permanent secretary Sir Tom Scholar told the House of Commons Public Accounts Committee that he had received a number of text messages from Mr Cameron in March and April last year and spoke to him by phone on 7 April. His second-in-command Charles Roxburgh confirmed that he held nine conference-call meetings with Greensill to discussion its request for access to Covid support funding and its proposals to act as a conduit for loans to businesses hit by the pandemic.

Challenged by committee chair Meg Hillier over whether other lobbyists would have found it as easy as Mr Cameron to get through to him, Sir Tom confirmed that it was his routine practice to take calls from ex-ministers and private sector figures he had previously worked with, adding: “If a former minister that I have worked with asks to talk to me, I will always do that … I think it’s natural when somebody that you know asks to speak to you, it’s quite natural to take that call.”

Scholar and Roxburgh confirmed that they were aware that Cameron was also making direct approaches to chancellor Rishi Sunak and Treasury ministers Jesse Norman and John Glen, but insisted they did not feel “under any pressure” to alter their response to Greensill’s requests for support under the Covid Corporate Financing Facility (CCFF) , which were eventually rejected.

But a series of MPs questioned the time and manpower devoted to discussions with the company. And Ms Hillier warned of “a danger of government by WhatsApp”, referring to the encrypted messaging system favoured by many in Westminster.

Meanwhile, the Bank of England released a 24-page dossier of emails and letters sent by Greensill, and Mr Cameron as its senior adviser, as the company sought approval for CCFF funds.

The records, released in response to a freedom of information request, showed that Mr Cameron emailed the Bank’s deputy governor Sir Jon Cunliffe on 5 March 2020, to set up a call on 17 March at which he and company founder Lex Greensill spoke to Bank officials.

Minutes of the call reveal that Greensill “explained that they were coming under significant pressure in current market conditions” and called for a re-establishment of a scheme used in the 2009 financial crisis to “protect the supply of working capital to the real economy”.

After the company was turned down for CCFF help, Mr Cameron – who was also contacting Mr Johnson and Mr Sunak on their mobile phones in a bid to reverse the decision – contacted Cunliffe on 3 April.

Signing himself “Dc”, the former prime minister said the Treasury seemed “hung up” on the fact that the CCFF was designed for non-financial corporates, insisting that this was “irrelevant” as Greenshill wanted to pass on loans to small and medium-sized enterprises (SMEs).

“At a time when we are – rightly – worried about how quickly banks can get loans out to small businesses, why are we potentially cutting off a market that already pumps cheap credit directly into SMEs?” he asked the deputy governor. 

“I think I must be missing something here. Am obviously talking to HMT, but would be grateful for any light you could shed on this.”

On 22 April, he told Cunliffe he still had not got the green light from the Treasury, describing the situation as “incredibly frustrating”.

In an apparent sign of the company’s increasing desperation, Mr Cameron said that Greensill’s capacity “remains severely constrained as many fixed income investors – who historically provide meaningful capacity – are currently remaining on the sidelines”.

A Bank memo on 24 April recorded that Mr Greensill spoke by phone with Sir Jon that morning, noting: “Most of the call was Lex explaining their business model. Jon was clear that Greensill would currently fall outside the boundaries of the scheme, and that expanding the parameters was a decision for HMT.”

Greensill’s attempts to secure state Covid support ultimately came to nothing and the company has since gone into administration.

A poll for The Independent today revealed the scale of public concern over cronyism and lobbying after a string of revelations, including the award of lucrative coronavirus contracts to firms with links to Conservative ministers.

The survey of 2,000 voters by pollsters Find Out Now found that 73 per cent believed action was needed to prevent public contracts going to cronies and 69 per cent said better controls to limit lobbying were needed, while 86 per cent said politicians should be more accountable for their actions and the same percentage said any who lied should lose office.