He told MPs: “The honest truth is… I cannot remember when I last spoke to Dave”.
But he insisted they had not talked about revelations that the ex-Tory leader lobbied the chancellor Rishi Sunak, including by text at one point, to include Greensill in the government’s Covid-19 loan scheme.
Mr Johnson also rejected Labour’s call for a wider inquiry into the lobbying scandal, claiming “it won’t do a blind bit of good”.
The prime minister ruled out a parliamentary inquiry, ahead of a Commons vote later today, as he defended an investigation he has ordered instead.
He admitted, however, that it was unclear whether or not the “boundaries” that are supposed to exist between Whitehall and business had been “properly understood”.
Yesterday it emerged that the head of Whitehall procurement became an adviser to Greensill Capital while still working as a civil servant, a move that was approved by the Cabinet Office.
At Prime Minister’s Questions, Mr Johnson said he shared the “widespread concern about some of the stuff that we’re reading at the moment”.
“I do think it is a good idea in principle that top civil servants should be able to engage with business and should have experience of the private sector,” Mr Johnson said.
“When I look at the accounts I’m reading to date, it’s not clear that those boundaries had been properly understood and I’ve asked for a proper independent review of the arrangements that we have to be conducted by Nigel Boardman, and he will be reporting in June.”
Labour claimed the Greensill affair marks the return of “Tory sleaze”.
Labour’s Rachel Reeves also accused Mr Cameron of a “cynical and shabby” decision to break his silence on the scandal last weekend during a period of “national grief” after the death of the Duke of Edinburgh.