Image copyright AP

Ex-Russian spy Alexander Litvinenko may have been poisoned with polonium "not once but twice", the public inquiry into his death has heard.

Mr Litvinenko died from radiation poisoning in a London hospital in November 2006, nearly three weeks after drinking tea laced with the substance.

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

The 43-year-old may also have been poisoned in October that year, counsel to the inquiry Robin Tam QC said.

Image copyright FACUNDO ARRIZABALAGA Image caption Mr Litvinenko's widow Marina and her son Anatoly spoke to reporters outside the Royal Courts of Justice on Tuesday

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

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," Mr Tam 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."

'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.

The judge-led inquiry was officially opened by presiding judge Sir Robert Owen at the Royal Courts of Justice.

Sir Robert said Mr Litvinenko's death from radiation poisoning in London in 2006 had attracted "worldwide interest and concern".

Mr Litvinenko's widow Marina says he blamed the Kremlin as he lay dying in hospital, but Russia denies any involvement.

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

Her lawyer has described his murder as "an act of state-sponsored nuclear terrorism on the streets of London".

Mr Litvinenko was poisoned with radioactive polonium-210, following a meeting with two Russians at the Millennium Hotel in central London.

The pair, Dmitry Kovtun and former KGB bodyguard Andrei Lugovoi - whom the UK police have identified as suspects in the case - have been invited to give evidence via videolink from Russia, Sir Robert said.

Mr Lugovoi told Russian television station LifeNews TV 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."

Many theories

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

The judge said the use of polonium could have killed large numbers of people "or spread general panic and hysteria among the public".

"The issues to which his death gives rise are of the utmost gravity and have attracted worldwide interest and concern," he said.

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?

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

"As we shall hear over the coming weeks, for some of these theories there is considerable supporting evidence, for others less, and for yet others none at all," he said.

Mr Tam said Mr Litvinenko and his family had fled Russia and their journey to the UK "would not disgrace the pages of a thriller".

They arrived in the UK in November 2000 and claimed asylum. Mr Litvinenko and his family were granted British citizenship just weeks before he died.

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

Mr Tam 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 inquiry heard other considerations included Mr Litvinenko's criticism of President Putin's regime.

"Could the Kremlin have regarded him as an irritant or worse?" asked Mr Tam.

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.

He asked: "If Mr Litvinenko was working for MI6, could this have become known in Russia and might this have provided a motive to anyone in Russia whether in authority or otherwise for wishing Mr Litvinenko dead?"

The inquiry heard one of the suspects, Mr Lugovoi, had alleged that the 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.