The UK government has proposed closing the book on most unsolved killings during the Troubles, prompting dismay from the Irish government and Northern Ireland’s nationalist parties.

A new independent body will review cases, and only those deemed to have compelling new evidence and a realistic prospect of prosecution will receive a full police investigation, the government announced on Wednesday.

The legislation would close the vast majority of almost 2,000 unsolved cases and prevent them being re-opened, marking a new approach to so-called “legacy” issues that have plagued politics and reconciliation efforts in Northern Ireland.

“Once cases have been considered there will be a legal bar on any future investigation occurring,” the Northern Ireland Office said. “This will end the cycle of reinvestigations for the families of victims and (army) veterans.”

Previously all or most cases were going to receive a full investigation, a process estimated to last a decade and cost £400m.

Families of victims in cases excluded from police investigations will receive “information” about the deaths, said Brandon Lewis, the Northern Ireland secretary. “I hope that by giving as many families as possible information on how their loved ones lost their lives, we can help ease the difficult process of reconciliation.”

Boris Johnson’s government has promised to table the legislation before the end of April.

The Irish government issued a tacit rebuke to the changes to the legacy approach agreed in 2014. “The Stormont House Agreement framework is the way forward,” said Simon Coveney, the foreign affairs minister. “It was agreed by both governments and the political parties after intensive negotiations, and it must be implemented.”

Michelle O’Neill, Northern Ireland’s deputy first minister and deputy leader of Sinn Féin, accused Downing Street of breaching previous commitments. “Some families have been waiting for up to 50 years for due process and the right to an … effective investigation and/or inquest.”

The SDLP leader, Colum Eastwood, said his party would resist what he called an act of “spectacular bad faith”.

Grainne Teggart, Amnesty UK campaigns manager for Northern Ireland, said:“These proposals are a further betrayal of victims already let down by the government’s failure to put in place mechanisms to deliver truth, justice and accountability.”

The revised legacy plans were published on the same day the government disclosed a plan to limit prosecutions against soldiers and veterans who have served abroad – which does not cover Northern Ireland.