Scotland Yard has contended that the Russian state was involved in the murder of Alexander Litvinenko as the force broke its silence at the inquiry into the spy’s death from poisoning.

Delivering an excoriating closing speech on the penultimate day of the public inquiry, lawyers representing the Metropolitan police tore into Russia’s record on cooperation with the investigation into Litvinenko’s murder.

Facebook Twitter Pinterest Alexander Litvinenko’s widow, Marina. Photograph: Frank Augstein/AP

Richard Horwell QC laid out a forceful character assassination of the key suspects in the killing – Russians Andrei Lugovoi and Dmitry Kovtun – describing the pair as an ignorant odd couple who were expendable to their masters.

Litvinenko, 43, a former FSB and KGB spy who worked for British secret service MI6 during his time in the UK, died at University College hospital in London nearly three weeks after drinking tea laced with the radioactive isotope polonium-210 on 1 November at the Millennium hotel in London’s Grosvenor Square. He had been in the company of Lugovoi and Kovtun.

Addressing Russian state involvement, Horwell told Sir Robert Owen, the judge presiding over the inquiry: “The evidence suggests that the only credible explanation is in one way or another the Russian state is involved in Litvinenko’s murder.”

Horwell detailed reasons why the Russians were likely to have been involved in the murder. From changes to Russian law in 2006 permitting the Kremlin to employ agents to kill extremists overseas to Litvinenko’s decision in the late 90s to expose corruption in the FSB and Russian government, Horwell said Russia had “reasons aplenty for wishing Litvinenko not only harm but death”.

Furthermore, he said, Russia had obstructed the police investigation and the public inquiry. “The attitude of the Russian state to this inquiry has been nothing short of contemptuous,” he said.

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Horwell alluded to speeches made by the former Soviet leader Joseph Stalin as he attempted to explain why the Russian state wanted Litvinenko dead. A quote attributed to Stalin, which roughly translates as “no men, no problem”, resonated with Litvinenko’s death, he said.

“The murder of Alexander Litvinenko was intended to solve the problem that he had become, but in reality it had created a much greater one and one which this inquiry has ensured will not go away.”

Horwell had remained silent until now, declining to cross-examine witnesses, because in the absence of any representation for Lugovoi and Kovtun or the Russian state, the Met did not want to be seen to have any influence over evidence.

Instead, he used his closing speech to make the force’s case against Lugovoi and Kovtun clear. “The Metropolitan police service want Lugovoi and Kovtun to be tried in this country for murder,” he said.

Horwell described in detail the trail of polonium left by Lugovoi and Kovtun across London and Hamburg.

From planes to hotel rooms, from a passport photo to a receipt for clothing, the list of places and items that came into contact with Lugovoi and Kovtun and were found to be contaminated with the radioactive substance was exhaustive, he said.

Facebook Twitter Pinterest Andrei Lugovoi. Photograph: Sergey Ponomarev/AP

“The Met investigation has always had at its core the science,” Horwell said. “Lugovoi and Kovtun have no credible answer to the scientific evidence and the trail of polonium they left behind.”

Horwell labelled Lugovoi and Kovtun “common murderers”. “Lugovoi and Kovtun weren’t bungling assassins as some have suggested. They were simply ignorant,” he said. “If Lugovoi and Kovtun had been aware of the true nature of this poison, they would have known that it would leave a radioactive trail.”

Horwell said the suspects’ ignorance suited those who planned Litvinenko’s murder.

“However important Lugovoi and Kovtun may think they are, to their masters they were, and are, quite simply expendable,” he said. “If they or their families had died, or if their life expectancies had been reduced, that to their masters would have been regarded as mere and acceptable collateral damage.”

The inquiry will conclude on Friday following closing speeches from Ben Emmerson QC, who represents Litvinenko’s widow, Marina, and their son, Anatoly, and Robin Tam QC, counsel to the inquiry. A report from Owen is expected before the end of the year.