London (CNN) A UK public inquiry into the 2006 death of Russian spy Alexander Litvinenko opened Tuesday at the Royal Courts of Justice in London, after years of wrangling over what evidence can be heard.

In a deathbed statement, Litvinenko blamed Russian President Vladimir Putin for ordering his poisoning by tea at a London hotel. The Kremlin has always strongly denied the accusation.

Sir Robert Owen, who's chairman of the inquiry, said Tuesday that sensitive material relating to possible Russian state involvement in Litvinenko's death would be heard behind closed doors.

The British government initially rejected requests to hold a public inquiry, but the decision was reversed last summer after Litvinenko's widow, Marina Litvinenko, challenged it in court.

She argued that a public inquiry would enable the fullest possible investigation.

An inquest -- a coroner-led investigation that is held as a matter of course in the case of unnatural deaths in England -- had been opened after her husband's death. But unlike a public inquiry, it cannot hear evidence behind closed doors.

In Alexander Litvinenko's case, such evidence could involve matters of national security.

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

Whistle-blower

Litvinenko, a former KGB agent and fierce critic of Putin, came to Britain in 2000 after turning whistle-blower on the FSB, the KGB's successor.

He died at a London hospital on November 23, 2006, after being poisoned by the radioactive material polonium-210 while drinking tea at the Millennium Hotel in London's Grosvenor Square.

UK prosecutors have asked for the extradition of two men, Andrei Lugovoi and Dmitry Kovtun, from Russia in connection with Litvinenko's murder.

But Moscow has refused, saying Russia's constitution does not allow the extradition of Russian citizens. Both men deny involvement in Litvinenko's death.

Owen said that Lugovoi and Kovtun had been invited to give evidence to the inquiry by video link from Russia and that he hoped they would do so.

The public inquiry will look at possible Russian state involvement in Litvinenko's death. However, it will not address the question as to whether the UK government could, or should, have taken steps to prevent the murder.

Litvinenko is said by his widow to have been a British agent, with a handler at MI6, Britain's foreign security service.

In 2012, the counsel to the inquest, Hugh Davies, said evidence provided by the UK government showed Russian involvement and "does establish a prima facie case as to the culpability of the Russian state in the death of Alexander Litvinenko."

Pathologists, nuclear experts to testify

The inquest has been put on hold while the public inquiry is held.

Owen, the coroner in the inquest, said the open hearings in the inquiry should conclude before Easter; that is, early April.

More than 70 witnesses are due to be called over the coming weeks, including family and friends of Litvinenko, those who worked with him before his death, medical staff who treated him after he fell ill and the pathologists who conducted his autopsy, the court heard.

The pathologists will testify Wednesday on the postmortem results. The presence of radiation in Litvinenko's body complicated the autopsy, the inquiry heard.

A nuclear scientist will also give evidence Wednesday about polonium-210, its qualities, where it can be found and what effect it has on the body once ingested.

In the course of the inquiry, evidence may also deal with the contamination risk posed to the wider public by the transfer of such highly radioactive material.

Owen said polonium could have been used to "kill large numbers of people or spread general panic and hysteria among the public."