Image copyright PA Image caption Former soldiers staged a protest outside the Ministry of Defence

Theresa May is facing demands from 150 Tory MPs and peers to drop plans to investigate past crimes in Northern Ireland and other military conflicts.

In a letter to the PM, they say a new Historical Investigations Unit would put "service and security personnel at an exceptional disadvantage".

And they accuse the government of breaking the Armed Forces Covenant - its manifesto commitment to personnel.

The Northern Ireland Office declined a request for comment.

Proposals for a Historical Investigations Unit were part of the 2014 Stormont House agreement and designed to deal with killings where there had been no prosecutions.

Last year, the Police Service of Northern Ireland (PSNI) said that investigations into killings by the Army account for about 30% of its Troubles legacy workload.

But concerns have been building in Westminster over whether military veterans have enough protection from unfair prosecutions, with some cases going back decades.

Wednesday's letter is signed by many former veterans, who are now MPs - including Johnny Mercer, Mark Francois and Richard Benyon - and former heads of the military including Lord Dannatt.

A group of MPs, also including former defence secretary Michael Fallon, delivered a petition to Downing Street on Tuesday urging action.

The new unit is meant to take over the work of the Historical Enquiries Team and legacy work of Northern Ireland's police ombudsman that remains outstanding.

But the Tory MPs and peers say the existing system is "completely at odds" with the government's commitment to the armed forces - and the proposal for a new unit would put "service and security personnel at an exceptional disadvantage".

The Ministry of Defence and the Northern Ireland Office have clashed over the plans in recent months and a consultation recently finished.

Now Attorney General Geoffrey Cox is understood to be looking at ways of limiting the likelihood of unfair prosecutions.

Cabinet sources have told the BBC that one proposal under consideration would be for potential prosecutions of former service personnel to require consent from the attorney general before taking place.

Image copyright PA Image caption The MPs are taking their case directly to Downing Street

There are concerns in government however about politicising the process in that way and about interfering in devolved legal matters.

Defence Secretary Gavin Williamson said: "I don't want to see elderly veterans having to face repeated investigations decades after the events in question. The Northern Ireland Office recently closed a consultation on legacy cases like this and will report soon.

"The Ministry of Defence has put in place a comprehensive package of support, including legal representation to any individual accused of an offence arising from their service on operations. A new team has also been created to consider the concerns of veterans and the wider public in order to find appropriate solutions."

Downing Street said it would respond "as soon as we can" to the public consultation, to which 16,000 people have so far responded.

The prime minister's spokesman said "everyone deserves the opportunity to be heard".

A statement issued on behalf of the attorney general said: "As a matter of convention, known as the Law Officers Convention, we do not disclose whether or not the attorney general or any of the law officers have given law advice or been asked for it."