Johnny Mercer’s appointment as minister for veterans has prompted Sinn Féin to voice concern that a Boris Johnson-led government would extend a proposed amnesty from prosecution for ex-soldiers to cover those who served in Northern Ireland.

Initial briefings suggested the new prime minister had told Mercer – who has campaigned for the amnesty to be extended to cover the Troubles – in a phone call to halt vexatious claims made against all former soldiers.

That led Francie Molloy, an MP for the Irish nationalist party, to declare Mercer’s elevation to be “further evidence” that the government intended to give “current and former British soldiers immunity from prosecution”.

There has been growing pressure in Conservative circles to halt historic prosecutions of military personnel following the collapse of the Iraq historic allegations team (Ihat) following allegations that claims had been dishonestly been brought forward.

Last week, the then defence secretary Penny Mordaunt unveiled a consultation aimed at introducing a presumption against prosecution for alleged offences committed in Iraq, Afghanistan and elsewhere outside the UK more than 10 years ago.

But it was careful to exclude Northern Ireland, which is part of a separate process to examine how to deal with the legacy of the Troubles, in which 3,500 people died over a three-decade period and many cases remain legally unresolved.

Mercer, a former army officer, had announced he was withholding support from Theresa May’s government until an end was brought to what he described as the “abhorrent process” of “elderly veterans being dragged back to Northern Ireland”.

The official announcement of Mercer’s appointment on Monday as minister for defence people and veterans was accompanied by plans to create an Office for Veterans’ Affairs “to deliver the lifelong support our veterans deserve”.

It contained no direct reference to plans for an amnesty, but Whitehall sources acknowledged that Mercer was privately briefing he had been told by Johnson that he would be able to examine the historic prosecutions issue.

“I am delighted with this role and am resolutely determined to reset this country’s relationship with her veterans. I entered parliament to do precisely this,” Mercer said.

In March, it was announced that one paratrooper, known only as Soldier F, would be prosecuted for murder and attempted murder following the deaths of 13 civil rights demonstrators on Bloody Sunday in 1972. Families representing victims said they were disappointed that more former soldiers had not been prosecuted.

About 150 to 200 army veterans and police officers are believed to be under investigation by the Police Service of Northern Ireland. Military cases account for about 30% of the workload, but only cover 10% of the deaths from that period.

Mercer will run the Office for Veterans’ Affairs with Oliver Dowden, the paymaster general, who will act as a minister representing veterans in the cabinet. The office will work across government but will be based in the Cabinet Office and staffed partly by officials from the Ministry of Defence.

Molloy added that the appointment of Mercer was “another calculated insult to victims of British state violence” and that any attempt to create a hierarchy “by protecting perpetrators of murders and other serious crime committed by British state forces are totally unacceptable”.