Watch Live: Final Stages Of Litvinenko Inquiry

The counsel for Marina Litvinenko is due to make his closing statements to the inquiry into her husband Alexander's death.

It comes after Scotland Yard detectives said they believe Mr Litvinenko had been poisoned twice.

The counsel for the Metropolitan Police Richard Horwell QC told the inquiry into the former spy's death on Thursday there was a failed attempt to kill him on 16 October 2006, followed a fortnight later by a successful attempt when polonium-210 was dropped into his tea at the Millennium hotel in London's Mayfair.

Mr Horwell, who was speaking as the public phase of the inquiry draws to a close, said police want to see the two prime suspects, Russians Andrei Lugavoi and Dmitry Kovtun, stand trial for murder.

"On the evidence, there can be no doubt that Litvinenko was unlawfully killed and the science is such that the finger points unwaveringly to Lugovoi and Kovtun," he said.

The two suspects have previously claimed Mr Litvinenko may have committed suicide, but Mr Horwell said: "(He) had everything to live for and was in good health. Devoted to his family and immensely proud of his son.

"Polonium is an exceptionally rare substance ... which is very difficult to acquire."

On claims by the pair that Mr Litvinenko was involved in the polonium trade, Mr Horwell said: "There is no evidence to support this fanciful theory ... it's the claim of desperate men and must be ignored.

"However many state honours Putin might pin to Lugovoi's chest for services to the motherland or however many times Kovtun promises to blow apart this inquiry, they have no credible answer to the science. It is untainted and damning."

Mr Horwell told the inquiry that many lives were put at risk when the two suspects decided to use a deadly radioactive poison on Mr Litvinenko.

"Anyone who arranges to bring polonium-210 into a city centre does so without any regard for human life," he said.

Addressing claims by the counsel for the Litvinenko family, Ben Emmerson QC, that the murder was effectively "a nuclear attack on the streets of London", Mr Horwell said: "That claim is justified.

"There is evidence that polonium may have been used in the past as the murder weapon of choice on other victims and had it not been identified on this occasion it would doubtless have been used again in the future."

Earlier Mr Litvinenko's widow said the six-month, London-based hearing had "shone a light" on the lengths the Kremlin will go to silence dissent.