REUTERS - The Russian state must have been involved in the 2006 poisoning of ex-KGB agent Alexander Litvinenko with a radioactive isotope, which amounted to "a nuclear attack on the streets" of London, an inquiry into the death was told on Thursday.

Kremlin critic Litvinenko died weeks after drinking green tea laced with polonium-210 at London's plush Millennium hotel. From his deathbed he accused Russian President Vladimir Putin of ordering his killing but the Kremlin has always denied any role.

"The evidence suggests that the only credible explanation is that in one form or another the Russian state was involved in Litvinenko's murder," Richard Horwell, the lawyer acting for London police, said in closing remarks to the British inquiry.

The controversy generated by Litvinenko's killing plunged Anglo-Russian relations to a post-Cold War low, and the inquiry chairman has already said there was prima facie evidence of Russian culpability.

Britain has accused Russians Dmitry Kovtun and Andrei Lugovoy, also a former KGB agent but now a lawmaker, of actually carrying out the poisoning, but they deny any involvement and Russia has refused to extradite them.

The inquiry has been told traces of polonium were found across London where the two men had been, including offices, hotels, planes and even the soccer stadium of Arsenal.

"No matter how many state honors Putin may pin to Lugovoy's chest for services to the mother ... or how many times Kovtun promises to blow apart this inquiry, Lugovoy and Kovtun have no credible answer to the scientific evidence and the trail of polonium they left behind," Horwell said.

Neither had any personal motive and were "common murderers", he added.

But the Russian state might want Litvinenko dead for many reasons such as his defecting to Britain where he was granted citizenship shortly before his death, his accusations about Kremlin corruption, his sympathy for Chechen separatists and his "explosive" claim that Putin was a pedophile - made in an online article in 2006 - Horwell said.

"There can be no doubt that the Russian state had reasons aplenty for wishing Litvinenko not only harm but death," he added.

Earlier, he said the use of the rare polonium, 97 percent of the world's supply of which was produced at a Russian nuclear site, might have put Londoners' lives at risk.

"We will never know how dangerous the exposure of polonium to the public at large will be and what long term effects will be visited upon Londoners", he said.

"Mr Emmerson (the lawyer representing Litvinenko's widow Marina) has said ... this was a nuclear attack on the streets of London. That comment is justified."

The inquiry's report is due by the end of the year.