A Conservative MP has withdrawn his support for Theresa May and her government in the Commons over the historical prosecutions of ex-servicemen and women.

In the latest blow to the PM’s authority, Johnny Mercer said it was “regrettable” that he could not continue to support the government, effectively going on strike.

The former Army officer said he found the repeated investigations into allegations – some dating back decades – “personally offensive”.

He said he was not prepared to vote for government legislation – except on Brexit – until the government took “clear and concrete steps” to end the “abhorrent process”.

In the letter, the Plymouth MP called on May to end “elderly veterans being dragged back to Northern Ireland” to face possible prosecution.

He wrote: “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.

“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 the government who sent them almost 50 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 has campaigned against the pursuit of legacy cases from the conflicts in Northern Ireland, Afghanistan and Iraq since he entered parliament in 2015.

He recently accused the Tory whips office of contacting former Army comrades in an attempt to dig up dirt on him.

He wrote in his letter: “It has not been an easy decision to make. But this incident with your chief whip has forced my hand.

“It appears that my values and ethos may be slowly, but very firmly, separating from a party I joined in 2015.”

There was no immediate response from Downing Street to his letter.