Tobias Ellwood has confirmed he has left his role as defence minister.
It comes as Boris Johnson continues to shake up his cabinet since becoming Prime Minister on Wednesday.
In a tweet, Mr Ellwood said: "It's been a privilege. I return to the backbenches with ever more passion, respect and humility for our amazing Armed Forces. And will continue to make the case for further defence spending."
The MP for Bournemouth East said he was "very sad" to see Penny Mordaunt leave the role of Defence Secretary and has backed her successor Ben Wallace to do a good job.
"I know that Ben Wallace will do an incredible job," he told Forces News on Thursday.
"He's a good friend, with Army experience. He is very thorough, he comes from just doing the security brief as well, so very understanding about what is happening across the world.
"I know that he will be passionate about securing more funding for the Armed Forces."
Speaking the day after his sacking, Mr Ellwood told BBC Radio 4's Today Programme it was a shock, describing politics as a "brutal game".
Before going into politics, Mr Ellwood spent six years in the Army with The Royal Green Jackets, serving in Northern Ireland, Cyprus, Germany, Gibraltar and Bosnia.
He was first elected as a Member of Parliament in May 2005 and began working as a defence minister in June 2017.
Mr Ellwood has been a strong supporter of mental health and, in the past, he has called on those in the Armed Forces to open up about their experiences.
In an exclusive interview, Mr Ellwood told Forces News his military training and experience gave him the confidence to step forward and help during the Westminster terror attack last year.
The reservist and former regular in the British Army was hailed as a "hero" after he tried to save the life of PC Keith Palmer who was stabbed to death while on duty outside the Palace of Westminster.
The former Army officer whose brother, Jonathan, was killed in the 2002 Bali terror attack described how he is "haunted" by the fact he could not save PC Palmer.