American intelligence agencies say ordered the killing. In a statement made in April, Mr. Cameron insisted that he took the opportunity to raise human rights issues.

On Thursday, Mr. Cameron explained his barrage of texts as a consequence of the urgency of the situation but conceded that, with hindsight, he should have made his approaches by formal letters or emails. He believed Greensill was offering good ideas to the government, Mr. Cameron said, denying that his lobbying was motivated by his financial interest.

Greensill pitched itself as an intermediary between the government and payees, offering to accelerate payments to businesses and individuals. In the case of individuals, Mr. Cameron defended the practice as a sort of populist alternative for some people to usurious payday-lending schemes. But the bulk of the lending was aimed at companies doing business with the government, and critics always questioned the wisdom of using an outside finance firm rather than simply speeding up government payments.

Professor Bale said that it was hard to think of any similarly overt lobbying of ministers from a former prime minister, not even Tony Blair, who was much criticized for his consultancy work.

“It is illustrative of a decline in standards because it used to be the case that this kind of thing ‘wasn’t done’ — and now it is,” Professor Bale said. The silver lining, he added, was that “the embarrassment caused to David Cameron might put some of his successors off.”

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