Media playback is unsupported on your device Media caption Ben Emmerson QC said Mr Litvinenko was murdered for exposing corruption

Ex-Russian spy Alexander Litvinenko was murdered for trying to "expose the corruption" at the heart of Vladimir Putin's "mafia state", the public inquiry into his death has heard.

His poisoning with polonium was an "act of nuclear terrorism on streets of a major city", said Ben Emmerson QC, the barrister representing his family.

Mr Litvinenko died in a London hospital in 2006 from radiation poisoning.

Two Russians, Andrei Lugovoi and Dmitry Kovtun, have denied any involvement.

Mr Litvinenko was 43 when he died from radioactive polonium-210 poisoning, three weeks after drinking tea laced with the substance at a meeting with the Russians at London's Millennium Hotel in November 2006.

In his opening statement on the first day of the inquiry, Mr Emmerson said Mr Litvinenko was "eliminated" because he had made an enemy of the "close knit group of criminals who surrounded and still surround Vladimir Putin and keep his corrupt regime in power".

Image copyright AP Image caption Mr Litvinenko started working with British security service MI6 in 2003, Ben Emmerson QC said.

Image copyright PA Image caption Ben Emmerson QC and Marina Litvinenko arriving at the inquiry

"It was an act of nuclear terrorism on the streets of a major city which put the lives of numerous other members of the public at risk," he said.

The murder was an act of "unspeakable barbarism" that inflicted "the most painful and lingering death imaginable", he added.

Alexander Litvinenko had fled to the UK where he became a vocal critic of the Kremlin and worked for the UK intelligence service MI6.

His widow Marina says he blamed the Kremlin as he lay dying in hospital, but Russia denies any involvement.

'No fair trial'

Mr Emmerson claimed there was not the "slightest doubt" that the murder was carried out by Mr Kovtun and former KGB bodyguard Mr Lugovoi.

The pair have been invited to give evidence via video link to the inquiry at the Royal Courts of Justice, led by judge Sir Robert Owen.

But Mr Lugovoi told Russian radio station Ekho Moskvy on Tuesday that there could be no fair trial in Britain.

"They classified the materials, saying Litvinenko co-operated with English intelligence. How can it be investigated impartially after that?" he said.

"This is why we pulled out in protest - we want it to be investigated but we want it to be impartial and, moreover, we want it investigated in Russia."

Image copyright Getty Images Image caption UK police identified two suspects in the case - Andrei Lugovoi and Dmitry Kovtun - but both deny any involvement and remain in Russia

The inquiry also heard that Mr Litvinenko may have been poisoned with polonium "not once but twice".

He had recalled feeling unwell around the time of a meeting at a security company in mid-October and "vomiting on one occasion about two or three weeks before being hospitalised," counsel to the inquiry Robin Tam QC said.

"Hair samples that are available indicate that Mr Litvinenko may well have been poisoned twice and that the first occasion being much less severe than the second."

One witness said he had been asked by Mr Kovtun to find a cook in London who could lace Mr Litvinenko's food with "expensive poison", Mr Tam told the inquiry.

'Worldwide concern'

BBC security correspondent Gordon Corera earlier said investigators had followed a radioactive trail across London.

It suggested Mr Litvinenko was poisoned not on the first attempt, but on the third, he said.

The two men suspected of killing Alexander Litvinenko made three trips to London in the run up to his death and brought Polonium to try to kill him each time, the BBC understands.

Sir Robert Owen said the death from radiation poisoning in London in 2006 had attracted "worldwide interest and concern".

Media playback is unsupported on your device Media caption Sir Robert Owen said sensitive evidence had established a "prima facie case" as to the culpability of the Russian state

He said sensitive evidence had established there was a "prima facie case" as to the culpability of the Russian state in Mr Litvinenko's death.

Sir Robert was originally appointed as the coroner at Mr Litvinenko's inquest but he called for a public inquiry because the inquest could not consider sensitive evidence due to national security fears.

The UK government resisted the move at first but later changed its stance last July, amid worsening relations with Moscow over the crisis in Ukraine.

The Litvinenko case

23 Nov 2006 - Mr Litvinenko, 43, dies three weeks after having tea with former agents Andrei Lugovoi and Dmitry Kovtun in London

24 Nov 2006 - His death is attributed to polonium-210

22 May 2007 - Britain's director of public prosecutions decides Mr Lugovoi should be charged with the murder of Mr Litvinenko

31 May 2007 - Mr Lugovoi denies any involvement in his death but says Mr Litvinenko was a British spy

5 Jul 2007 - Russia officially refuses to extradite Mr Lugovoi, saying its constitution does not allow it

May-June 2013 - Inquest into Mr Litvinenko's death delayed as coroner decides a public inquiry would be preferable, as it would be able to hear some evidence in secret

July 2013 - Ministers rule out public inquiry

Jan 2014 - Marina Litvinenko in High Court fight to force a public inquiry

11 Feb 2014 - High Court says the Home Office had been wrong to rule out an inquiry before the outcome of an inquest

July 2014 - Public inquiry announced by Home Office

January 2015 - Inquiry starts. Judge expects to conclude open hearings by early April and report before end of 2015

BBC security correspondent Gordon Corera: Will inquiry find answers?

Mr Tam said many theories had been put forward about what happened to Mr Litvinenko, including suggestions that he had killed himself, or accidentally poisoned himself when handling the radioactive substance as part of a smuggling deal.

He listed numerous issues which would have to be considered by the inquiry, such as Mr Litvinenko's relationship with the late Russian businessman Boris Berezovsky and his links to journalist Anna Politkovskaya, a fierce critic of Russian President Vladimir Putin who was killed in 2006.

The QC said evidence would be heard - both in the open and closed sessions - that Mr Litvinenko had been working for MI6 - a claim which the government refuses to confirm or deny.

The inquiry heard Mr Lugovoi had alleged UK intelligence services were involved in Mr Litvinenko's death.

It will also hear evidence that Mr Litvinenko could have been assisting the Spanish security services with investigations into organised crime, as well as taking on private security work for Western businesses.