Rishi Sunak, the chancellor, has denied giving special treatment to the now-collapsed bank Greensill Capital following an intensive lobbying campaign by former prime minister David Cameron.

Sunak repeatedly denied that he and his team were swayed by receiving 25 text messages, 12 WhatsApps, 11 phone calls and eight emails from Cameron lobbying on behalf of Greensill, the bank for which the former prime minister worked and in which he had an undisclosed multimillion pound equity stake.

The chancellor said the collapse of the bank could end up costing the taxpayer at least $8m (£5.5m) and possibly as much as $400m in other indirect costs. However, Lord Myners, the former City minister, has said the demise of the supply chain financing firm could cost taxpayers £5bn.

Sunak told MPs on parliament’s Treasury select committee on Thursday that he only spent “a very small amount of time” considering Cameron’s proposals and ultimately rejected Greensill’s request to be part of the Bank of England’s Covid corporate financing facility (CCFF) scheme.

“I looked at the issue on the merits of it, so the identity of the person talking about it was not relevant to the amount of attention and proper due diligence that the issue got and required,” Sunak said. “This was one of many strands of work, and in fact probably the one we spent the least time on during this period.”

However, Mel Stride, the committee’s Conservative chairman, told Sunak that his denials that he was swayed by being contacted by Cameron were not credible.

“It just doesn’t seem credible if it was a former prime minister pushing something as vigorously as he did, at the very highest level,” Stride said.

Sunak replied that Cameron’s lobbying on behalf of Greensill was considered only because “we were dealing with a financial crisis [for] small- to medium-sized businesses and this was a proposal that potentially addressed a segment of that market”.

The chancellor said receiving the messages from Cameron had come as a “surprise” as he was not friends with the former PM.

“I don’t know David Cameron very well at all and I don’t think I have spoken to him since I was a backbench MP and he was PM, so it was a surprise to receive the message,” he said.

In one of the messages, Cameron said to Sunak: “Rishi, David Cameron here. Can I have a very quick word at some point. HMT are refusing to extend CCFF to include supply chain finance … There is a simple misunderstanding that I can explain. Thanks DC.”

Asked if he would do things differently in future, Sunak said: “No, I don’t. As I said, I stand very firmly behind the approach we took.”

Siobhain McDonagh, a Labour MP on the committee, told Sunak she found it hard to believe that insistent lobbying from Cameron did not have an effect on his decision-making. She reeled off the list of messages including “25 texts, 12 WhatsApps, eight emails, 11 calls and nine meetings with senior ministers and officials”.

“Can you name me another company that got that amount of access at the height of the crisis?” she asked.

In text messages sent to Cameron, Sunak told the former prime minister that he had “pushed the team” at the Treasury to explore ways that Greensill could join the government’s Covid-19 loan support scheme. Sunak denied that this suggested he was putting pressure on Treasury or Bank of England officials to help Cameron and Greensill.

“It’s a really common phrase I would use on an almost daily or weekly basis, by talking about work that is being worked on in the department,” Sunak said. “It’s nothing more than a phrase.”

The Treasury’s two top civil servants also denied that Cameron’s lobbying resulted in preferential treatment for Greensill. Charles Roxburgh, the Treasury’s second permanent secretary, said the collapse could cost the government $8m and rejected Myners estimate of up to £5bn.

“I do not know how Lord Myners came up with that number, we don’t recognise it,” he said. “You will have to ask Lord Myners how he arrived at it.”