Cameron, who stepped down after losing the Brexit referendum in 2016, is accused of exploiting his relationship with ex-government colleagues for his own benefit.
The former United Kingdom prime minister has been at the center of Britain's biggest lobbying row in a generation after it emerged he pressed senior ministers and officials to include Greensill Capital - a supply-chain finance specialist that employed him as an adviser after he left governent - in a coronavirus lending scheme.
Cameron is scheduled to appear before the Treasury select committee on Thursday.
He insisted there was "absolutely no wrongdoing" in his lobbying attempts, but accepted that former prime ministers must "act differently".
He told MPs he had a "big economic investment" in the future of Greensill, but refused to state how much he was paid yearly by the firm as he insisted it was a "private matter".
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Cameron's salary as prime minister was around 150,000 pounds ($210,500; €174,300).
Cameron responded: "I was keen to get it in front of Government, but as I've said, there are lessons to learn and lessons for me to learn and in future the single formal email or formal letter would be appropriate".
Official documents show that Mr Cameron and his staff sent numerous emails, texts and WhatsApp messages to top government ministers between March 5 and June 26 previous year.
But MPs have seen evidence of dozens of text messages and emails sent by Mr Cameron to ministers and officials in Whitehall and the Bank of England.
The ex-PM came armed with his own modest proposals for reforming Britain's lax lobbying rules, which, despite changes introduced by Cameron's own government, cover only a small subset of agency lobbyists and miss the vast swath of corporate influencers working directly for companies.
Mr Cameron told the Treasury Committee he had not broken any rules when he tried to influence ministers and officials on behalf of Greensill Capital around the start of the Covid pandemic in spring previous year.
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L'auteur présumé d'un double meurtre dans les Cévennes ce mardi matin est toujours activement recherché par les gendarmes. La gendarmerie a laissé un numéro de téléphone (04 66 38 67 22) et déconseille vivement d'intervenir seul.
"I made a choice to work for a business which I hoped would be a United Kingdom fin tech (financial technology) success story", he added, calling its collapse "deeply regrettable".
"I did not believe in March or April previous year when I was doing this contact there was a risk of Greensill falling over", he said.
What happens to Greensill Capital?
Greensill collapsed earlier this year, leaving 3,000 jobs at a steel manufacturer at risk, and prompting the U.K.'s Financial Conduct Authority to launch an investigation into the "potentially criminal" circumstances of its failure.
The scheme was initially financed by Greensill's former employer Citigroup, but that role was was taken over by Greensill capital in July 2018, when Cameron's ally Theresa May was PM.
"I don't have any money offshore, I have an office of David Cameron which I fund through some of the work that I do, and I earn some money in a personal capacity".
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Cameron said his lobbying was motivated by a desire to help support British workers and businesses "in the economic turmoil caused by COVID" and not by the prospect of making millions from his Greensill shares.