Home Secretary Jack Straw has defended the UK authorities' decision not to arrest suspected Nazi war criminal Konrad Kalejs.

Mr Kalejs, a Latvian-born Australian citizen, left the UK on Thursday for Australia, pre-empting deportation proceedings brought against him.

Campaigners in Australia say they hope to bring him to trial on his return, while the Latvian authorities have called for international help with their investigations into Mr Kalejs' past.

Mr Straw said the police had insufficient evidence to arrest Mr Kalejs who is alleged to have been an officer in a notorious Nazi death squad responsible for the murder of 30,000 people during World War II.

The home secretary told the BBC he was satisfied with the police's judgement of the case and said due to the separation of powers it was not his job to "second guess" their decision.

'Complicit' in war crimes

Explaining the reasoning behind Mr Kalejs deportation, Mr Straw said: "I am satisfied on the balance of probabilities that this man was complicit in war crimes.

"Therefore it is conducive to the public good for me to institute deportation proceedings against him."

Mr Straw said he understood the "frustration" of those campaigning for Mr Kalejs' arrest, but he said the 86-year-old's departure was an ironic consequence of living under the rule of law, which afforded alleged war criminals rights they would not have given their victims.

But Mr Kalejs denies he was the second-in-command of the notorious Arajs Kommando, responsible for murdering thousands of Jews in Latvia.

'Deeply disappointed'

Veteran war crimes investigator Lord Janner is now calling on the Australian government to properly investigate Mr Kalejs.

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 UK had evidence

Responding to the news that Mr Kalejs has left the UK Dr Anthony Glees, reader in politics at Brunel University, said that a "very, very fat" dossier on the alleged war criminal had been in the possession of Scotland Yard.

Dr Glees told the BBC: "I know there's a whole lot of evidence on him at New Scotland Yard because 10 years ago I was the adviser to the war crimes inquiry in the Home Office."

He added: "I think this is an outrageous decision that gives entirely the wrong signal to former Nazi war criminals and any other war criminal and any other human rights abuser."

Kalejs' file remains 'open'

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.

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."