The new defence secretary has promised to introduce an amnesty on historical prosecutions for military veterans who served in Iraq, Afghanistan and anywhere else around the world – with the exception of Northern Ireland.

Penny Mordaunt will consult on proposals for a presumption against prosecution for offences committed more than 10 years ago and will say she supports plans to opt out of the European convention on human rights (ECHR) in future armed conflicts.

But the minister risks courting conflict with some on the right of her party, who want Northern Ireland to be included within any amnesty, following the prosecution of a former paratrooper for the murder of two people on Bloody Sunday in Derry in 1972.

Mordaunt said: “It is high time that we change the system and provide the right legal protections to make sure the decisions our service personnel take in the battlefield will not lead to repeated or unfair investigations down the line.”

A ministerial statement detailing the announcement is expected within the next few days and the defence secretary could be questioned about the plans when she attends the Sea Power conference in central London on Wednesday morning.

Pressure has been mounting in Conservative circles on the issue. Earlier this month, the Tory backbencher Johnny Mercer said he would refuse to support the government in the Commons until legislation on the issue was brought forward.

The former head of the army, General Lord Dannatt, said peers would try to amend the legislation to extend it to Northern Ireland when it comes to the House of Lords.

“What we can’t allow to go forward is the presumption that those deaths in which the military were involved were wrong,” he told the BBC Radio 4 Today programme. “Soldiers did their duty, got up in the morning, sometimes they came under attack. They returned fired. They didn’t set out to murder people. Terrorists set out every morning to murder people and successfully did so. There is a huge distinction to be drawn.”

Last week, Mordaunt hinted that she would act and has pursued proposals that were being worked up by her predecessor Gavin Williamson, who was fired in early May after being accused of leaking sensitive material to a journalist.

Mordaunt is also expected on Wednesday to reaffirm a controversial commitment to opting out of the ECHR before the start of “significant military operations”, where the government deems it appropriate.

The original commitment was made by Theresa May at a Conservative party conference in 2016 and was intended to see off “industry of vexatious claims” against soldiers that the Ministry of Defence has costed at more than £100m since 2004.

Derogating from the ECHR in times of war or public emergency is permitted under the rules of the Council of Europe, which oversees the Strasbourg-based institution. But the idea has been criticised by the human rights group Liberty, which argues that most legal claims are not vexatious and relate to offences that cannot be derogated from, such as torture.

Countries that have derogated include Ukraine, which gave notice of a derogation in June 2015, in relation to the fighting on its border with Russia. France signalled that it would derogate in the immediate aftermath of the jihadist massacres at the Bataclan nightclub in Paris last November. Turkey lodged a similar notice following the failed military coup in July.

Historic prosecutions dating back more than 10 years would be allowed if there were “exceptional circumstances” where compelling new evidence emerged, but the effect of the measure will be to largely exclude future claims dating from the Iraq wars.

Combat operations in Iraq were declared to have ended in 2009 following the 2003 invasion that led to the fall of Saddam Hussein and troops pulled out in 2011. Combat operations in Afghanistan, which began in 2002, ended in 2014.

Mercer, a former army officer, has written to May to say that the “historical prosecution of our servicemen and women is a matter that is personally offensive to me” and demanded action from the prime minister.

The MP for Plymouth Moor View said he would refuse to support the government over non-Brexit-related legislation until an end was brought to what he described as the “abhorrent process” of “elderly veterans being dragged back to Northern Ireland”.

Concerns about introducing an amnesty to Northern Ireland, where 3,500 people died during the Troubles and many cases remain legally unresolved, had prompted Sinn Féin to warn there should be “no immunity or impunity for British forces guilty of crime, collusion and murder in Ireland”.

The Northern Ireland Office is conducting its own consultation into the handling of legacy cases. The task of investigating any unresolved murder cases is largely dealt with by the legacy investigations branch (LIB) of the Police Service of Northern Ireland.

Last month, the Guardian revealed that between 150 and 200 army veterans and police officers are believed to be under investigation. Military cases account for around 30% of the LIB’s workload, but only form 10% of the overall deaths from that period.