The revelation that former Defence Secretary Liam Fox broke the ministerial code of conduct has dominated prime minister's question time.
Labour leader Ed Miliband led with the topic on 19 October 2011 telling MPs the news was "deeply worrying".
Mr Fox resigned from the government last week, saying he had blurred personal and professional boundaries over his dealings with close friend and self-styled adviser Adam Werritty.
A report by Cabinet Secretary Sir Gus O'Donnell found there had been a "failure of judgement", for which Mr Fox took ultimate responsibility by resigning.
In the Commons, Mr Miliband said: "The former defence secretary had an unofficial adviser with access to top officials in the military and indeed foreign governments, funded by undeclared private donations solicited by him.
"Yet the prime minister said he and No 10 knew nothing about these goings on for 18 months. How did he allow this to happen?"
David Cameron agreed it was a serious issue which was why he had asked Sir Gus to carry out a "full and proper inquiry".
But, he added: "I do think it is worth actually recognising that in this case the secretary of state for defence has recognised that he had made a mistake, acknowledged that he broke the ministerial code and he resigned. That is not something that always happened in the last 13 years."
Mr Miliband demanded to know the full extent of ministerial contacts with Mr Werritty. He wanted a "categorical guarantee" from the prime minister that no other government minister had engaged in similar activities over the past 18 months.
Mr Cameron reiterated that Mr Fox has resigned and accusing him of being "a bit late" to jump on the bandwagon on the issue.
Mr Miliband used the remainder of his questions to focus on the economy, asking Mr Cameron whether he believed his plan was still working.
Mr Cameron accused him of talking down the economy and said Labour had no plan to deal with the country's debts.