The widow of poisoned Russian spy Alexander Litvinenko has said the "whole world" will know the truth about what happened to her husband as a public inquiry into his death begins today.

Coroner Sir Robert Owen suspended the current inquest into Mr Litvinenko's death before opening the inquiry, which was announced by Britain's Home Secretary Theresa May last week.

In his opening statement, Mr Owen praised Mr Litvinenko's widow, Marina Litvinenko, for her patience in the face of "highly regrettable" delays.

Ms Litvinenko said today was a "special" day and she was confident the inquiry will start on schedule, adding: "Everybody all around the world will know the truth."

The 43-year-old spy was poisoned with radioactive polonium-210 while drinking tea at the Millennium Hotel in London's Grosvenor Square in 2006.

On his death bed Mr Litvinenko accused President Vladimir Putin of ordering his killing.

The case threatens to increase tensions with Moscow amid the crisis in Ukraine.

When asked if she thought last week's announcement was due to the strained political situation with Russia, Ms Litvinenko said the political situation may have been a contributing factor, but she did not think there had been any "political involvement" in the process.

She added that she had always been aware parts of the inquiry would have to be in secret, and admitted she may never know the content of some of the evidence submitted to the inquiry.

Ms Litvinenko said she was reassured it would be viewed by the chair and other lawyers.

The hearing is to be adjourned until early next year, when Mr Owen is expected to start taking evidence, a spokesman said.

The inquiry is expected to last until the end of 2015.

The British government said the judge would be able to look at whether the Russian state was behind the mysterious killing, which outraged the UK at the time and had a negative impact on relations with Moscow.

The UK has strenuously denied any link between its decision to launch the inquiry and international pressure on Russia over the downing of Malaysia Airlines flight MH17 over Ukraine.

However, the decision is a major turnaround for the British government.

It comes just months after it had resisted attempts to hold any inquiry on the grounds of protecting sensitive information about Russian and British intelligence.

Russia's ambassador to London, Alexander Yakovenko, last week said that Moscow would not accept the judgment of the inquiry if any of the evidence was given in secret.

Mr Litvinenko, 43, an ex-agent in Russia's FSB intelligence agency who turned against his former masters, was poisoned with radioactive polonium-210 while drinking tea at a London hotel.

In a deathbed letter, Mr Litvinenko said he believed Russia's president was involved in his killing after he publicly criticised the leader, himself an ex-Soviet KGB agent.

British police have identified Russian spy-turned-politician Andrei Lugovoi as the chief suspect and have issued an arrest warrant for his fellow former agent Dmitri Kovtun, but Moscow has refused to hand them over.

They both deny involvement, while Mr Lugovoi called the inquiry politically motivated.

British Home Secretary Theresa May originally wanted to wait for the results of a separate inquest into Mr Litvinenko's death.

In English law, inquests are held to examine sudden, violent or unnatural deaths. While they determine the place and time of death as well as how the deceased came by their death, they do not apportion blame.

But three High Court judges ruled in February that Ms May must reconsider that decision, following a challenge by Mr Litvinenko's widow Marina.

Mr Owen must formally suspend the inquest today before opening the inquiry.