During a review of the newspapers on BBC TV on Sunday morning Mr Downer, now Australia's High Commissioner in London, said he'd met Nemtsov and found him to be "such a fine man, really he was a very impressive man".

However he predicted the "ghastly" murder could be quite damaging to president Vladimir Putin and his regime.

"Not because there's any evidence that he was behind this assassination but it creates a sense of instability in the country and uncertainty and there will be a lot of sympathy for Nemtsov.

"I don't think it's a good day for President Putin, although not suggesting he knew anything about it … Putin's claim to authority is that he's keeping the country stable. But in fact what's happening here is an act of huge instability."

Australia's ambassador to Russia, Paul Myler, said on Twitter that he would not be in Moscow for Sunday's Nemtsov mourning march "but I'll be there in spirit".

"I hope today's march commemorating Boris Nemtsov is allowed to be peaceful, respectful and an overwhelming rejection of violence," he said.

After the assassination on Friday Mr Myler said Nemtsov was a "political survivor". The murder was depressing, he said, adding "[There] has to be a better way."

Mr Downer was also asked about the unfolding story about ISIS executioner 'Jihadi John', now identified as Londoner Mohammed Emwazi.

He said the "larger motif" of a story in the Daily Mail, in which Emwazi complained to a Mail reporter that he suspected MI5 had bought his old laptop, was that "the security services are doing their job".

"This is as true in my own country as it is here … they do a huge amount of work intercepting threats and saving people's lives and they don't get a lot of credit for it.

"My experience of MI5 here is that it's an extraordinarily professional organisation. Of its kind it would be the most professional in the world."

UK security services have been criticised in recent days for being aware of Emwazi but failing to stop him leaving the country to join Islamic State.