Johnny Mercer MP has resigned as Minister for Defence People and Veterans.
In his letter to Prime Minister Boris Johnson, Mr Mercer had "no choice" but to leave the Government over frustrations regarding the lack of legislation to protect British veterans who served during the Troubles.
Mr Mercer, a former British Army officer, has been heavily involved in the Overseas Operations (Service Personnel and Veterans) Bill.
The proposed legislation, which is going through its final stages in Parliament, does not include Northern Ireland in its scope.
In his resignation letter, he stated that Northern Ireland veterans "deserve the protections of the Overseas Operations Bill like everyone else".
Mr Mercer said he was "sorry" to leaving his ministerial position and told the Prime Minister he was doing so "with a heavy heart".
"I am very proud of the small team in the Office for Veterans Affairs who have worked hard against a strong prevailing wind in Government to establish themselves and start the significant piece of work of getting the UK Government to realise her responsibilities to those who have served in the UK's Armed Forces," he wrote to Mr Johnson.
"I am, of course, desperately sad events have transpired the way they have – I truly have exhausted my efforts and my team to make it work.
"But the truth is politics always was a means to change how this country treats her military veterans, and I remain genuinely appalled by the experiences of some of this nation's finest people who have served in the Armed Forces.
"I fought and bled alongside them, I've been far more fortunate than many of them since, and I have a duty to tell their truth to power."
I’m sorry to have been relieved of my responsibilities in Government tonight. pic.twitter.com/AWai59fVhE— Johnny Mercer (@JohnnyMercerUK) April 20, 2021
He added that "endless reinvestigations" involving veterans meant he felt: "We have abandoned our people in a way I simply cannot reconcile."
Mr Mercer ended the letter by stating that not including those who served during the Troubles in legal protections for veterans was his "red line", adding: "I am deeply proud of my predecessors who served in Northern Ireland.
"They are not second-class veterans. They deserve the protections of the Overseas Operations Bill like everyone else.
"I made promises on your behalf that we would not leave them behind and would walk through simultaneous legislation for them.
"No discernible efforts have been made to do so, and I can see no prospect of this changing.
"I have no choice but to leave Government and campaign for them in Parliament."
Johnny Mercer spent 10 years in the British Army and served in Afghanistan with 29th Commando Regiment in the Royal Artillery.
Two months after leaving the military he became the Conservative candidate for Plymouth Moor View and was elected in 2015.
Mr Mercer was made a Parliamentary Under Secretary of State in both the Ministry of Defence and the Cabinet Office in 2019.
He was responsible for the welfare of Armed Forces personnel as part of the role and oversaw the Office for Veterans' Affairs.
A Downing Street spokesperson said the Prime Minister "thanks Johnny Mercer for his service as a Government minister since 2019".
Mr Johnson said the Overseas Operations Bill is a "crucial part" of efforts to protect personnel against "vexatious and repeated" legal claims.
"But we are committed to doing more over the coming months, including for those who have served in Northern Ireland," he added.
Cover image: UK Parliament