Liam Fox put the security of both himself and Ministry of Defence officials at risk by providing details of his diary to his close friend Adam Werritty, the cabinet secretary has said.
In a report published on Tuesday, Sir Gus O'Donnell said Fox allowed “an inappropriate blurring of lines between official and personal relationships” and had committed a "clear breach" of ministerial code.
"The disclosure outside MoD of diary details about future visits overseas posed a degree of security risk not only to Dr Fox, but also to the accompanying official party," Sir Gus said.
"Dr Fox has accepted that such disclosures were not appropriate. Clearer and stricter guidance will be issued within MoD about revealing to third parties any information about ministers’ future commitments, travel plans and accommodation arrangements, including in cases where the minister wishes such details to be disclosed.
"I am of the view that this is an issue which was specific to Dr Fox. However, I will ensure that this guidance is made available to other departments."
Fox was heavily criticised for meeting up with Werritty on 18 separate occasions in different cities around the world since May 2010.
Werrity got his friend Fox in to trouble after he passed himself off as an official adviser to the defence secretary, even though he held no official government or Conservative Party post.
While the report cleared Fox of the two most serious allegations, namely that he had gained financially from the situation or that national security had been breached, it is unlikely he would have been able to survive in office had he not resigned on Friday.
In a statement issued following the publication of the report Fox said: "As I said in the House of Commons last week, I accept that it was a mistake to allow the distinctions between government and private roles to become blurred, and I must take my share of the responsibility for this," he said.
"More care should have been taken to avoid the impression that anyone other than minsters and officials were speaking on behalf of the government, as this was not the case. Although there were no actual conflicts of interest I acknowledge that in order to avoid any possible perception of this, all private interests should have been fully declared to the permanent secretary.
"I welcome the recommendations in this report which will provide greater clarity for ministers, officials and private individuals in the future."
Downing Street said the government would seek to implement the recommendations of Sir Gus' report.
"This government has already introduced changes that significantly increase government transparency - publishing lists of meetings with external organisations and all procurement over £500," it said.
"In this case, the MOD permanent secretary has accepted that there should have been much tighter procedures within the department and is taking steps to strengthen them to ensure that the ministerial code is properly adhered to in future.
"The cabinet secretary has now recommended further strengthening of procedures across government.
"The prime minister accepts the cabinet secretary's recommendations and the cabinet secretary will write to permanent secretaries setting this out."
But Labour's Jim Murphy, the shadow defence secretary, said the report from Britain's most senior civil servant left too many questions unanswered.
“A ten page report into eighteen months of wrongdoing is a superficial and narrow way for the Government to deal with such a deep problem," he said.
“This report only scratches the surface of potential wrongdoing. This is a murky business and it has not yet been resolved."
He added: “We need to know the role and influence of Liam Fox’s ‘unofficial adviser’, the nature of any solicited donations and the full extent of Adam Werritty’s funding and the access he had."