Sir Michael Fallon's resignation leaves Theresa May facing the sensitive task of replacing him in the Cabinet.
Here are some of the potential contenders for the role of Defence Secretary:
:: Sir Alan Duncan: currently Boris Johnson's deputy in the Foreign Office, Sir Alan was at Oxford University at the same time as Mrs May - and preceded her husband Philip as president of the Oxford Union. The former oil trader was instrumental in setting up a covert cell to block fuel supplies to Libyan leader Muammar Gaddafi's regime while ensuring rebels had access to the resource during the 2011 conflict.
:: Penny Mordaunt: The Work and Pensions Minister has experience in the Ministry of Defence and is a Royal Navy reservist. She was the first female Armed Forces Minister from May 2015 until July 2016. The Portsmouth North MP donned her swimsuit to appear on ITV diving competition Splash in 2014 to raise money for charity.
:: Rory Stewart: A former soldier, diplomat and writer, the Penrith and the Border MP's life has been so colourful that Brad Pitt's production company reportedly bought the rights for a biopic. Currently Minister for Africa, Mr Stewart had a short stint as an officer in the Black Watch before a diplomatic career which saw him become deputy governor of two provinces in Iraq following the 2003 invasion. On leave from the Foreign Service he walked for 21 months crossing Iran, Afghanistan, Pakistan, India and Nepal, staying in 500 village houses on the journey. Mr Stewart's name appeared on the unverified list of sexual misconduct allegations circulating in Westminster but said the claims about his behaviour towards a female member of staff were "completely untrue", something the researcher publicly backed up.
:: Ben Wallace: The Security Minister and a former officer in the Scots Guards, he was promoted to his Home Office role when Mrs May took over in Number 10. But Mrs May could be wary about putting Boris Johnson's former campaign manager in the Cabinet alongside the Foreign Secretary at a time when her grasp on the leadership has been weakened by the general election humiliation.
:: Tobias Ellwood: Currently a junior minister in the Ministry of Defence, he served in the Royal Green Jackets from 1991 to 1996 with tours in Northern Ireland, Cyprus, Kuwait, Germany, Gibraltar and Bosnia. He was praised for his efforts trying in vain to save the life of Pc Keith Palmer after the Westminster terror attack in March.
:: Johnny Mercer: An outside bet for the role, but if the Prime Minister wanted to revitalise her administration, the former army officer and Afghanistan veteran could offer that option. Only elected in 2015, he impressed Westminster observers with his tenacious leadership of a Defence Select Committee investigation into the handling of the Iraq Historic Allegations Team. As a critic of Mrs May's welfare policies, the Prime Minister may also want to get him inside her Government tent in order to remove a backbench rebel. The Plymouth Moor View MP once appeared half-naked lathering himself up in an advert for Dove shower gel.
:: Tom Tugendhat: Another member of the 2015 intake, the Tonbridge and Malling MP has already become chairman of the Foreign Affairs Select Committee. A former military officer, he rose to the rank of lieutenant colonel during a career which saw him serve in Iraq and Afghanistan. He has been tipped as a future Tory leader.