In a letter to Prime Minister David Cameron, Dr Fox said that he had “mistakenly” allowed the distinction between his personal interest and government activities to become “blurred”. “I have also repeatedly said that the national interest must always come before personal interest. I now have to hold myself to my own standard,” he said. “I have therefore decided, with great sadness, to resign from my post as Secretary of State for Defence - a position which I have been immensely proud and honoured to have held.” In his response, Mr Cameron paid tribute to the “superb job” which Dr Fox had done at the Ministry of Defence. “I understand your reasons for deciding to resign as Defence Secretary, although I am very sorry to see you go,” he said. “We have worked closely for these last six years, and you have been a key member of my team throughout that time.” The announcement comes after days of damaging speculation over the relationship between Dr Fox and his 33-year-old best man and former flatmate. Mr Werritty was questioned for a second time today by a senior official from the Cabinet Office as part of the Whitehall investigation headed by Sir Gus O’Donnell. In full: Liam Fox’s resignation letter to Prime Minister David Cameron Dear David, As you know, I have always placed a great deal of importance on accountability and responsibility. As I said in the House of Commons on Monday, I mistakenly allowed the distinction between my personal interest and my Government activities to become blurred. The consequences of this have become clearer in recent days. I am very sorry for this. I have also repeatedly said that the national interest must always come before personal interest. I now have to hold myself to my own standard. I have therefore decided, with great sadness, to resign from my post as Secretary of State for Defence - a position which I have been immensely proud and honoured to have held. I am particularly proud to have overseen the long overdue reforms to the Ministry of Defence and to our Armed Forces, which will shape them to meet the challenges of the future and keep this country safe. I am proud also to have played a part in helping to liberate the people of Libya, and I regret that I will not see through to its conclusion Britain’s role in Afghanistan, where so much progress has been made. Above all, I am honoured and humbled to have worked with the superb men and women in our Armed Forces. Their bravery, dedication and professionalism are second to none. I appreciate all the support you have given me - and will continue to support the vital work of this Government, above all in controlling the enormous budget deficit we inherited, which is a threat not just to this country’s economic prosperity but also to its national security. I look forward to continuing to represent my constituents in North Somerset. Yours ever, Liam