Tory MP Johnny Mercer says he has withdrawn his support for Theresa May and her government over the historical prosecution of servicemen and women.
In a letter to the PM, the Plymouth MP said he would only vote with the Conservatives on Brexit legislation.
He called on Mrs May to end the “abhorrent process” of “elderly veterans being dragged back to Northern Ireland” to face possible prosecution.
He has previously called for legislation to stop this happening.
Communities Secretary – and former Northern Ireland Secretary – James Brokenshire said he was “very saddened” by Mr Mercer’s announcement and acknowledged that “the system isn’t working well in Northern Ireland”.
He said the government had been consulting on changing the existing system.
Speaking to BBC Radio 4’s Today programme, Mr Mercer said the government had “singularly failed to act for four years and I am simply not prepare to put up with it any more”.
“There is nothing loyal about watching the car go over the cliff and not doing anything about it,” he added.
In his letter the former Army officer and member of the Commons Defence Committee, said: “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.”
BBC political editor Laura Kuenssberg said the Conservative Party can “ill afford to lose MPs from [the] rising generation who have been able to win marginal seats” right now.
Mr Mercer told the PM he cannot “support your legislative programme any further until your government make some clear and concrete steps to end this abhorrent process”.
“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,” he wrote.
“It appears that my values and ethos may be slowly, but very firmly, separating from a party I joined in 2015.
“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 and will be updating my constituents of this decision accordingly.”
A total of six former soldiers are now facing prosecution over Troubles-era killings.
The cases relate to Daniel Hegarty; Bloody Sunday; John Pat Cunningham; Joe McCann (involving two ex-soldiers); and Aidan McAnespie.
Not all the charges are murder.
The Public Prosecution Service said that of 26 so-called legacy cases it has taken decisions on since 2011, 13 related to republicans, eight to loyalists, and five are connected to the Army.