London has become one of the most dangerous cities in the world for critics of Russia’s president, Vladimir Putin, with “no one safe” from the reach of the Kremlin, according to one of Alexander Litvinenko’s closest confidants.

Yuri Felshtinsky, a historian who co-wrote the book Blowing Up Russia with the former secret agent, said the capital had gone from being a safe haven for Russian expatriates to being dangerous for opponents of Putin.

Speaking hours after the closing statements of the inquiry into the killing of Litvinenko on Friday, Felshtinsky said that a string of unsolved murders indicated that a culture of impunity existed in London for the murderers of Russian dissidents.

“The message is that no one is safe in London. I do not know if everybody is safe in the rest of the world, but the message at this point is that no one is safe in London,” he told the Observer. Felshtinsky’s book with Litvinenko accused the FSB, the contemporary name for the KGB, of blowing up Moscow apartment blocks – killing nearly 300 people – to justify a war against Chechnya.

On Wednesday, it emerged that MI6 had judged the book one of two “red lines” Litvinenko crossed that caused him to be murdered by the Russian state. The other was allegations published by Litvinenko online in July 2006, four months before he was murdered, in which he claimed that Putin was a paedophile.

The book itself was recently added to Russia’s list of “extremist materials”, which means that online access to the title for anyone in Russia is restricted, with those in possession of a digital copy facing being penalised.

On Friday, the closing statements of the six-month inquiry into the poisoning of Litvinenko alleged that Putin personally ordered the killing of the 43-year-old dissident. Ben Emmerson QC, for Litvinenko’s family, said that Russian state responsibility had been proven beyond reasonable doubt.

Litvinenko drank tea containing a fatal dose of radioactive polonium during a meeting on 16 October 2006 at the Millennium hotel in central London with chief suspects Dmitry Kovtun and Andrei Lugovoi. Detectives found 40 sites contaminated with polonium-210.

Litvinenko family blames Putin for murder - vidoe Guardian

Felshtinsky, who lives in the US, said: “London traditionally was the safest place for Russian émigrés and this all started with individuals like Alexander Herzen, a famous Russian writer, a political dissident living in London from 1852 to 1864 – yet no one was trying to poison or kill him. Unfortunately in recent years London has become a very dangerous place for Russians.”

A series of high-profile mysterious deaths includes that of Alexander Perepilichny, 44, who was helping to investigate corrupt tax officials in Moscow when he died suddenly outside his Surrey mansion and was later found to have a poison in his body favoured by Russian assassins. “We know he was poisoned but not by whom, but there are reasons to believe it was done by the Russian governmentand there are still questions over Berezovsky,” said Felshtinsky.

Exiled oligarch Boris Berezovsky died in uncertain circumstances in Surrey in 2013 after falling foul of Putin, and amid claims he was about to hand the Russian president evidence of a coup plot.

Questions also remain over the death of Berezovsky’s business partner, Badri Patarkatsishvili, who died in February 2008, while last week “Putin’s banker” Sergei Pugachev revealed how he had fled from London to France because someone was trying to murder him.

Felshtinsky said: “London is very popular among Russian émigrés, but the fact that somehow you are able to organise a killing of a person in London and for this to go unpunished is a bad indicator, it creates a culture of impunity.”

Felshtinsky said he was not considered a traitor by the Russian state, unlike Litvinenko, and that was why he was not killed. “There is a different scale of crime in the eyes of the Kremlin committed by me or Litvinenko,” he said.

“We can write a book together, but he is a former officer who committed treason while I am just a writer. This is the difference. This is why he is dead and I am alive.”