Jeremy Corbyn has insisted that British troops must face investigation for alleged abuses carried out in Iraq and Afghanistan as Tony Blair added his voice to mounting criticism of the process.
Former prime minister Mr Blair said he was "very sorry" that soldiers were being put through such an "ordeal", and said the process looking into claims should never have been set up.
But Mr Corbyn said the UK had signed up to international law and it would be a "step too far" to say troops should not face prosecution for their actions.
Defence Secretary Sir Michael Fallon has pledged legal support for soldiers affected.
The Iraq Historic Allegations Team (Ihat), which investigates claims of murder and abuse by British soldiers in Iraq, has come under fire for its handling of allegations.
Concerns have been raised about false accusations being made and the length of time taken to address claims.
Mr Corbyn told BBC One's Andrew Marr Show: "I have spoken to a number of soldiers who served in Afghanistan and Iraq. I recognise the awful conditions they were asked to serve under and the difficulties they had with that.
"But I do think there has to be a recognition that we have signed up to international law on the behaviour of troops."
He said the USA and other European countries were going through the same experience.
"So I think there has to be investigation," he said.
"Saying never to prosecute, I think, would be a step too far."
Mr Blair told the Sunday Telegraph: "I do not think this process should ever have been put in place.
"I am very sorry that our soldiers and their families have been put through this ordeal."
He added: "Our armed forces gave extraordinary service in both Iraq and Afghanistan and this type of investigation simply makes their job harder to do."
Former soldiers have said they have been hounded through the courts on unfounded claims and there are growing calls for Ihat to be shut down.
At a meeting with defence chiefs on Thursday, Prime Minister Theresa May said "every effort" would be made to stop the legal system being abused in such cases and to protect forces personnel from "vexatious" allegations.
Sir Michael insisted that Ihat must continue its work if Britain was to avoid being brought before the International Criminal Court, but said troops facing investigation would be properly supported.
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