Veteran war crimes investigator Lord Janner has said he is "deeply frustrated" that alleged Nazi war criminal Konrad Kalejs was allowed to leave the UK.

Lord Janner called for the Australian government to properly investigate Mr Kalejs after the 86-year-old left the UK on board a Singapore Airlines flight on Thursday morning.

The flight was bound for Singapore but Mr Kalejs, a Latvian-born Australian passport holder, is expected to continue his journey to Australia.

By fleeing the country, Mr Kalejs, who had been staying at a retirement home in Leicestershire, pre-empted deportation proceedings started by Home Secretary Jack Straw earlier this week.

Mr Kalejs denies he was the second-in-command of the notorious Arajs Kommando, responsible for murdering more than 30,000 people, mostly Jews, in Latvia during World War II.

Speaking to the BBC, Lord Janner said: "The police took the view that there was not enough evidence.

"I am not in a position to say whether that is right or wrong. But I am deeply disappointed, frustrated and sad that this evil man was allowed to leave Britain before he was sent to jail for the rest of his days.

"It is now up to the Australian authorities. We are going to try and get them to investigate fully."

The Australian government has said that it cannot stop Mr Kalejs returning to the country as he holds an Australian passport. But the investigation into his background could be reopened if new evidence was uncovered.

Philip Flood, Australia's High Commissioner in London, said the file on Mr Kalejs would remain open and Latvia could seek his extradition.

He said: "In November 1997 Latvia sought documents concerning Kalejs and this material was provided to Latvia in April 1998 - very extensive material was provided.

"Since then we haven't had any request from Latvia for his extradition."

Further inquiries

As Mr Kalejs left the UK, campaigners in Australia said new evidence unearthed during deportation proceedings against the suspected war criminal in the United States and Canada could be used to bring him to trial under Australia's War Crimes Act 1945.

Gary Herz, from the Australia, Israel and Jewish Affairs Council, said: "This evidence was successful in forcing him to leave these countries and could be brought before a court of law in Australia which we believe could lead to his conviction."

Calls for more inquiries into Mr Kalejs came after the former head of the now disbanded Australian war crimes unit Robert Greenwood told the BBC on Wednesday that no "responsible investigative body" had ever examined the case with a view to bringing a prosecution.

Files from the former Soviet Union could provide extra evidence against Mr Kalejs and there was no reason why a full investigation should not lead to his being charged, he said.