Former head of the British Army General Lord Dannatt has defended Gavin Williamson's performance as defence secretary.
Mr Williamson was sacked by the Prime Minister on Wednesday evening over "compelling evidence" he was responsible for leaking details from a National Security Council meeting to a newspaper.
He has "strenuously" denied being responsible.
Royal Navy reservist Penny Mordaunt has replaced him, becoming the first ever female Defence Secretary.
Speaking to Forces News, General Lord Dannatt said he was "very disappointed" at hearing Gavin Williamson had been sacked.
"Whichever way you look at it, it's a personal disaster for Gavin Williamson."
He went on to describe the situation as "very upsetting and disturbing for the MOD".
"It's a difficult portfolio, he came 18 months ago knowing virtually nothing about defence, in fairness to him, he listened and he learned," General Lord Dannatt continued.
"He particularly picked up the fact that defence was underfunded and was vigorous in conducting discussions with Philip Hammond, the Chancellor, to try and increase the resources for defence.
"But, as we know he was prone to put his foot in it from time to time and I’m afraid on this occasion, it would seem he has put his foot in it big time."
General Lord Dannatt praised Penny Mordaunt as a good appointment, but acknowledged Mr Williamson "made a difference".
"Sir Michael Fallon had on his desk a list of very unattractive cuts in order to make financial savings to balance the Ministry of Defence's budget," General Lord Dannatt said.
"Michael Fallon didn’t like those things, and Gavin Williamson didn’t like those things either.
"He fought pretty hard to try and turn some of those cuts around."
Earlier on Thursday, General Lord Dannatt told BBC Radio 4's Today programme he spoke to Mr Williamson on the phone on Wednesday night following his sacking, where the former defence secretary "very much protesting his innocence".
"He has to protest his innocence over this, otherwise he is laying himself open to a potential criminal prosecution."
Gavin Williamson was appointed defence secretary in November 2017, following the resignation of Sir Michael Fallon.