Conservative plans to water down the Human Rights Act – to prevent prosecutions of soldiers accused of murders in Northern Ireland – will make the UK a pariah, the party has been warned.

The move could also lead to Britain leaving the European Convention on Human Rights (ECHR) altogether, at huge cost to the country’s reputation, legal experts said.

The backlash came after Boris Johnson pledged to end what the Tories call “unfair trials”, by banning inquests from returning verdicts of unlawful killings for deaths during the Troubles.

It would involve amending the Human Rights Act – the key legal route for families seeking to prove British state involvement in killings – to exclude any death in Northern Ireland before the legislation came into force in October 2000.

But Dominic Grieve, the former attorney general, now running as an independent, attacked a confusing announcement that he suggested was simply “electioneering”.

“I am very sensitive to soldiers not being harassed about events that happened a long time ago, but the rule of law has to be upheld as well,” he told The Independent.

Amnesty International said: “All victims have the right to an independent investigation – that is a cornerstone of the rule of law throughout the world.”

And Mark Stephens, a solicitor specialising in human rights, said: “This sounds like clickbait for Tory voters.

“The UK has been a signatory of the European Convention on Human Rights since 1958 and if we want to remain part of that convention any amendment of domestic legislation will have to be compliant with it.”

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The Tory pledge follows a long campaign by veterans’ groups, which have protested that the law is being abused to hound retired soldiers years after the events in question took place.

But, under Article 2 of the ECHR, nations are obliged to carry out an effective official investigation into deaths where lethal force has been used against individuals by agents of the state.

Investigations using the inquest system have been used by families to try to prove that their loved ones were killed unlawfully.

Mr Grieve added: “If we seek to stop inquests, we may fall foul of Article 2 of the European Convention on Human Rights. And if we seek to interfere with prosecutions, well, I’m staggered that any government would consider it.”

He warned it could lead to leaving the ECHR altogether, adding: “That would be a very bad destination indeed, because we are one of the leading countries seeking to apply it to improve standards, not just in Europe, but around the world.”

Grainne Teggart, Amnesty International’s Northern Ireland campaign manager, said: “It is essential that no one, including members of the armed forces, is above the law.

“Yet in preventing former soldiers from being prosecuted over killings and other abuses that took place during the Northern Ireland conflict, that is exactly where this would place them.”

Simon Coveney, the Irish foreign affairs minister, also criticised the plan, tweeting: “There is no statute of limitations, no amnesty for anyone who committed crimes in Northern Ireland.