Michelle Bachelet, the United Nations High Commissioner for Human Rights, urged lawmakers to ensure that British law remained entirely unambiguous when it comes to accountability for international crimes.

The prosecution of British soldiers for alleged past crimes in Northern Ireland, and in more recent conflicts in Iraq and Afghanistan, has dogged the military, government and courts for years.

The bill has been approved by the British parliament's lower House of Commons and is about to come before the upper House of Lords, which can bring in revisions and amendments.

Bachelet voiced concerns that, unless amended, the bill could lead to shielding troops operating abroad from due accountability for acts of torture or other serious international crimes.

"As currently drafted, the bill would make it substantially less likely that UK service members on overseas operations would be held accountable for serious human rights violations amounting to international crimes," the former Chilean president said.

The legislation discourages the prosecution of current or former soldiers for alleged offences committed on overseas operations more than five years ago.

It raises the threshold prosecutors will use in deciding to pursue a case after five years to "exceptional", and requires them to weigh the public interest and get consent from the attorney general before prosecuting.

Bachelet called on Britain to specifically exclude all international crimes from the proposed restrictions and to strengthen UK courts' ability "to resolve the most serious allegations against military personnel".

Britain's Defence Secretary Ben Wallace has claimed the bill will protect service personnel and veterans from "vexatious claims and endless investigations".

The British government said military operations will continue to be governed by other international humanitarian law, and denied the bill amounts to an "amnesty" for British troops.