A Conservative MP and former army officer has announced he will not vote with the government on any issue besides Brexit until new laws are brought forward to protect war veterans from prosecution over historical allegations.

Johnny Mercer wrote to Theresa May on Wednesday evening to criticise “repeated investigations with no new evidence” and “the macabre spectacle of elderly veterans being dragged back to Northern Ireland” as support for the prime minister continues to ebb.

He said his qualified withdrawal of support for the party had not been “an easy decision to make”, but that a recent episode with government whips had eroded his remaining sense of “residual cooperation”.

The MP for Plymouth Moor, speaking on the BBC Radio 4 Today programme on Thursday, said: “I’ve made it clear I voted for Brexit and I will vote for it again.

“But this is a huge issue in communities like mine that I’ve tried time and again over the last four years to do something about.

“What we do in this country is once our servicemen and servicewomen finish service, anyone can come along and investigate allegations with usually no evidence at all.

“There is one thing that stands between these people and this process, and that is the government who have failed to act for four years.

“I am not prepared to stand for it any more. I cannot retain my integrity and vote for this government.”

Mercer cited the case of Phil Shiner, a human rights lawyer who was struck off for bringing abuse claims against UK troops after the Iraq war.

The MP also denied his stance was due to other disagreements with the Conservative leadership, adding: “There’s nothing loyal about watching the car go over the cliff and not doing anything about it.”

Johnny Mercer MP (@JohnnyMercerUK) I have written to the Prime Minister. pic.twitter.com/H3lJKI6oA5

In his letter which was posted on his Twitter account, Mercer wrote: “Regrettably, I cannot continue to support your government whilst one particular injustice continues unabated, and the recent behaviour of your whips’ office has ground out of me any residual co-operation.

“As you know, the historical prosecution of our servicemen and women is a matter that is personally offensive to me. Many are my friends; and I am from their tribe. I worked hard to expose the Iraq Historical Allegations Team in 2017 and your government had no choice but to shut it down.”

He added: “These repeated investigations with no new evidence, the macabre spectacle of elderly veterans being dragged back to Northern Ireland to face those who seek to re-fight that conflict through other means, without any protection from a government who sent them almost fifty years ago, is too much.

“I will not be voting for any of the government’s legislative actions outside of Brexit until legislation is brought forward to protect veterans from being repeatedly prosecuted for historical allegations.”

Mercer, touted as a dark horse for the Tory leadership, has campaigned against the pursuance of investigations into wars in Iraq, Northern Ireland and Afghanistan – where he served as an officer – since he become an MP in 2015.

He celebrated the closure of the Iraq Historic Allegations Team, in direct response to a parliamentary inquiry that he chaired, as the best day of his career. However, he continued to criticise the government over similar investigations into alleged criminal actions perpetrated during conflicts.

Last month Mercer posted a redacted message he had received – which he said was from an old army contact – warning him to “watch his back”, since the whips’ office was trying to “dig up dirt” about his military career.

He wrote on Twitter days later: “The individual concerned in my case has been caught fair and square, and knows he has, as does the chief whip. A hands up apology will make it go away; childish lines denying it – from Downing Street or the government whips office – will end badly.”

Up to 200 former members of the British security forces are under investigation over historical accusations arising from the Troubles era, during which time a number of unarmed people were killed by the British army.