Click here to catch up on the key findings of the inquiry into the death of Alexander Litvinenko

And click here if you want to read all about polonium-210 - the radioactive substance used to murder him

Mrs May's response does not go anywhere near far enough in answering the inquiry findings - and risks being seen as weak, Mr Burnham says. 

He calls for a review of all of the UK's relations with Russia.

Will the government consider expelling all FSB officers from the UK, he asks.

Has the PM ever raised this case directly with Russian President Vladimir Putin, and is he seeking an urgent conversation now, he adds.

Marina Litvinenko says she has not given up on the killers of her husband being brought to justice. "I am motivated to find the people who killed my husband," she says.

"Today we received a verdict saying 'Yes, it was those two guys definitely who committed this crime'.

"Yes, they are walking the streets of Moscow today but I believe there will be a final day when these two people will be punished."

There are other Russian and former Russian citizens living in the UK who may also be at risk, Mr Burnham says.

Will the home secretary review security of those most at risk, he asks.

Shadow home secretary Andy Burnham, responding for Labour in the Commons, calls the murder "an unparalleled act of state-sponsored terrorism".

He says it is important to take time to consider the findings properly.

He also pays tribute to Marina Litvinenko for her campaign for justice for her late husband.

After previously ignoring the UK report's finding that Putin "probably" approved Alexander Litvinenko's killing, Russian state media have now alluded to it, but in a highly modified and weakened form.

A correspondent's report from London on 24-hour news channel Russia-24 said the report had claimed that the killing was "possibly" approved by the FSB, its then head, Nikolai Patrushev, and "allegedly even" President Putin. The word "possibly" is repeated several times through the report. (Via BBC Monitoring)

Russia's "continued failure" to ensure the perpetrators are brought to justice is "unacceptable", Mrs May says.

She says she has written to the UK's director of public prosecutions to ask what can be done in terms of extradition and freezing assets.

The government remains committed to persuing justice, she says.

The UK will express its "profound displeasure" to the Russian ambassador, she says.

The Treasury has agreed to impose asset freezes against the two prime suspects in the Litvinenko murder, Mrs May told the House of Commons. "Senior representations" were being made to the authorities in Moscow, while the Russian ambassador was being summoned to the Foreign Office.

Home Secretary Theresa May pays tribute to Marina Litvinenko and her "tireless efforts to get to the truth".

She says the government takes the findings extremely seriously, and are considering them in detail.

It was a "blatant and unacceptable breach of the most fundamental tenets of international law and civilised behaviour", she says.

BBC News Channel

BBC assistant political editor Norman Smith says government sources have been at pains to stress that action has already been taken following the murder - including the expulsion of four Russian diplomats.

However, the government will come under "serious pressure" in the Commons, with Labour demanding stronger action, he says.

Those calls are likely to be echoed by Lib Dems, he says.

Given the conflict in Syria, the UK government will have "no wish to further dent diplomatic relations" because the UK will need to work with Russia to fight so-called Islamic State, he says.

You can watch the statement here:

http://www.bbc.co.uk/iplayer/live/bbcparliament

David Cameron's official spokeswoman said Downing Street was taking the findings "extremely seriously" and that the prime minister found them "extremely disturbing". 

The spokeswoman said: "The conclusion that the murder was authorised at the highest levels of the Russian state is extremely disturbing. 

"It is not the way for any state, let alone a permanent member of the UN Security Council, to behave.

"Regrettably, these findings confirm what we and previous governments already believed." 

The spokeswoman said measures taken against Russia in 2007 remained in place, adding: "In the light of the inquiry's findings, we are considering what further action we should take."

Alexander Litvinenko's son Anatoly tells journalists: "It's been pretty difficult, I guess. It's the only life I've lived. It hasn't been easy, but I can't compare it with anybody else's life."

Ben Emerson QC, the lawyer for Alexander Litvinenko's family, says he cannot see the two men accused of his death being extradited to the UK to face trial any time soon.

"The only circumstances I can imagine is after the fall, inevitable as it is, of Vladimir Putin.

"He won't send them here for trial, because he sent them here to commit the murder."

He says he would very much like to see a new administration in Russia take action in the future.

Russia's Foreign Ministry says the inquiry was not impartial and claims the conclusions were pre-determined.

In a statement, Maria Zhakarova said: "We regret that a purely criminal case has been politicised and has darkened the general atmosphere of bilateral relations".

She added that "clearly the decision to suspend the coroner's inquest and begin 'public hearings' was politically motivated".

• Naturally occurring radioactive material that emits highly hazardous alpha (positively charged) particles
• First discovered by Marie Curie at the end of the 19th Century
• There are very small amounts of polonium-210 in the soil and in the atmosphere, and everyone has a small amount of it in their body
• At high doses, it damages tissues and organs
• It cannot pass through the skin, and must be ingested or inhaled into the body to cause damage
• Historically called radium F, is very hard for doctors to identify

Mr Litvinenko's widow, Marina Litvinenko, is giving a press conference in central London.

She has reiterated her calls for David Cameron to take action, including imposing a travel ban on Mr Putin and sanctions on Russia.  

She also thanked the Met Police, her legal team and the judge "who provided this justice".

She added a tribute to her son: "Anatoly was only 12 when his father died, and it was not easy for him to step forward and talk to the press. I'm so proud of him and I couldn't have done this without him."