Rishi Sunak texted David Cameron to say he had "pushed the team" at the Treasury to explore helping a company that employed the former Prime Minister, it emerged on Thursday night.
The Chancellor last year sent two messages to Mr Cameron in relation to Greensill, a finance firm advised by the ex-Tory leader that was seeking access to Government coronavirus support funding.
Both were released after a Freedom of Information request was submitted. The disclosure comes amid a fierce lobbying row over the collapsed lender.
Mr Sunak said he had "voluntarily published the communication" sent to Mr Cameron in the "interests of transparency".
A government source said that since the texts were sent to and from his personal mobile rather than a ministerial device, the FOI rules were unclear about whether he was obliged to publish the messages.
In a letter to the Labour shadow chancellor, Anneliese Dodds, Mr Sunak also confirmed that Mr Cameron had "reached out informally by telephone" to three Treasury ministers in total about Greensill's access to Covid funding – himself, the financial secretary Jesse Norman and the economic secretary John Glen.
The Treasury said it was withholding the texts sent to Mr Sunak by Mr Cameron as they were written in his capacity as an employee and "with an expectation of confidence".
Greensill had approached Treasury officials regarding access to the Covid Corporate Finance Facility, which was administered by the Bank of England.
Mr Sunak said officials looked at requests made by Greensill to change the terms of the scheme or broaden its scope to allow the lender to access it. Both proposals were rejected, as officials determined they were "not workable and would not deliver sufficiently" for UK small businesses, the Chancellor stressed.
However, he insisted that "it is right that as an institution the Treasury engages with stakeholders and considers policy suggestions that are put to us, especially in an unprecedented crisis".
According to the documents published on Thursday night, Mr Sunak said in his first text message to Mr Cameron on April 3, 2020: "Hi David, thanks for your message. I am stuck back to back on calls but will try you later this evening and if gets too late, first thing tomorrow. Best, Rishi."
In a second message, sent on April 23, he said: "Hi David, apologies for the delay. I think the proposals in the end did require a change to the Market Notice but I have pushed the team to explore an alternative with the Bank that might work.
"No guarantees, but the Bank are currently looking at it and Charles [Roxburgh, the second most senior mandarin at the Treasury] should be in touch. Best, Rishi."
Greensill filed for insolvency last month, putting at risk thousands of jobs in the UK steel industry. Mr Cameron reportedly told friends he was in line to make $70 million from share options if the lender had been publicly floated.
He has drawn heavy criticism following reports that, during his administration, he allowed Lex Greensill, the founder of the company, access to 11 Whitehall departments. Mr Cameron accepted a job at Greensill after he left the Government.
Mr Cameron has been exonerated by the lobbying watchdog, which ruled that, as an employee of Greensill rather than a freelancer, he was not obliged to declare his activities. He has not commented publicly on the row.
Ms Dodds said the text messages "raise very serious questions about whether the Chancellor may have broken the Ministerial Code", adding: "They suggest that Greensill Capital got accelerated treatment and access to officials, and that the Chancellor 'pushed' officials to consider Greensill's requests."
She said a full and transparent investigation must be launched into the chain of events that saw "Greensill awarded lucrative contracts, the freedom of Whitehall and the right to lend millions of pounds of Government-backed Covid loans".
The Telegraph attempted to reach a representative of Mr Cameron for comment.