Rishi Sunak "pushed" officials to find alternative ways to help a financial services firm David Cameron was lobbying for, text messages released by the chancellor reveal.
The chancellor said the former prime minister "reached out informally by telephone" to him about getting COVID support for Greensill Capital in April last year.
Mr Cameron also called Economic Secretary John Glen and Financial Secretary Jesse Norman about access to the COVID Corporate Finance Facility (CCFF), a Freedom of Information request has revealed.
Mr Sunak said the meetings covered requests made by Greensill to change the terms of the scheme or expand its scope to allow them access to cheap loans from the Treasury.
However, after "appropriate consultations" both requests were rejected.
Greensill has since collapsed, putting thousands of steelmaking jobs in the UK at risk and rendering Mr Cameron’s reported tens of millions of share options worthless.
The chancellor said it was right that the "Treasury listened to – and gave due consideration to – all potential options to support businesses to survive the pandemic".
He then confirmed Mr Cameron’s lobbying activities, saying the former PM reached out "informally" to him by phone and the matter was referred to the relevant officials.
Mr Sunak also released two text messages he sent to Mr Cameron in April 2020, but messages sent by Mr Cameron were withheld as the government said he sent them in his capacity as a Greensill employee "and with an expectation of confidence".
The first message from Mr Sunak, sent on 3 April 2020, said: "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"
A second text, sent on 23 April, 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 should be in touch. Best, Rishi"
"Charles" refers to Charles Roxburgh, second permanent secretary at the Treasury and "the Bank" is the Bank of England.
As a result of Mr Cameron’s involvement and his personal texts to members of the government, Labour has called on the government to tighten the law on lobbying.
The former Conservative leader has been exonerated of any wrongdoing over the issue.
The Registrar of Consultant Lobbyists said Mr Cameron was an employee of Greensill so was not required to declare himself on the register of consultant lobbyists.
Shadow Chancellor Anneliese Dodds said the latest text messages raise "very serious questions" about whether Mr Sunak broke the ministerial code.
She said: "They suggest that Greensill Capital got accelerated treatment and access to officials, and that the chancellor ‘pushed’ officials to consider Greensill’s requests.
"The chancellor’s decision to open the door to Greensill Capital has put public money at risk."
She called for a "full, transparent and thorough investigation" into how Greensill was awarded "lucrative contracts, the freedom of Whitehall and the right to lend millions of pounds of government-backed COVID loans".