The use of the rare chemical polonium to kill ex-KGB agent Alexander Litvinenko might have put Londoners' lives at risk, the inquiry into his death was told.
Richard Horwell, the lawyer acting for London police, has said the Russian state must have been involved in the 2006 poisoning of Litvinenko with a radioactive isotope, which amounted to "a nuclear attack on the streets" of London.
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.
"And 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 on Londoners. Anyone who arranges for polonium-210 to be brought into a city centre, does so without any regard for human life," Horwell said.
"[Litvinenko's widow Marina's lawyer] Mr Emmerson has said, perhaps it was more than once, that this was a nuclear attack on the streets of London, that comment is justified. London was plunged into crisis and the scale of the Metropolitan Police Service's response was considerable.".
Britain has accused Russians Dmitry Kovtun and Andrei Lugovoi, also a former KGB agent but now a lawmaker, of carrying out the poisoning, but they deny any involvement and Russia has refused to extradite them.
"Those who planned Litvinenko's murder did not want the cause of his death to be discovered. Polonium is a silent, invisible and normally unidentifiable agent of death," Horwell said.
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 football stadium of Arsenal. Some 97% of all polonium is produced at a Russian nuclear site.
"The Kremlin cannot exactly complain if the eyes of the world look to it for responsibility for Litvinenko's murder. And for all Litvinenko's targets Putin was the one most frequently in his sights," Horwell said.
Neither Kovtun nor Lugovoi had any personal motive and were "common murderers", he added.
"What on earth does Russia have to hide and why these impediments to the truth. The evidence suggests that the only credible explanation is that, in one form or another, the Russian state was involved in Litvinenko's murder," he added.
The inquiry's report is due by the end of the year.