Two Russians accused of launching a nerve agent attack on Britain may already be dead, a leading Vladimir Putin critic has claimed.

Andrei Piontkovsky believes that men - named by British police as Alexander Petrov and Ruslan Boshirov - could have been executed to hide traces of the alleged crime.

He compared the case to that of Andrey Lugovoy and Dmitry Kovtun, the men accused by Britain of poisoning Alexander Litvinenko with polonium in 2006.

Lugovoy and Kovtun went public to deny the claims soon after being accused, meaning the Russian authorities then protected them, said Piontkovsky.

Alexander Petrov (left) and Ruslan Boshirov (right) have been accused by British police of being two Russian spies who launched a novichok attack in Salisbury earlier this year

Police have demanded that Russia hand over the two men (pictured in Britain), but a leading critic of Putin believes they may already be dead

Andrei Piontkovsky, a political analyst, raised the prospect that Russia may have disposed of Petrov and Boshirov in order to hide evidence of the alleged crime

'Lugovoy and Kovtun rescued themselves by running to Ecko (radio station) and going public,' the respected mathematician and political analyst said.

'One (Lugovoy) even had to be made an MP. If 'Petrov' and 'Bashirov' don't appear in the coming days, it means they are already dead.'

British authorities say that Petrov and Bashirov are likely aliases used by two Russia GRU agents as they came to Britain armed with a perfume bottle filled with novichok in order to kill turncoat spy Sergei Skripal and daughter Yulia.

Britain and its allies have accused the Russian state of sanctioning the attack and say President Putin bears ultimate responsibility for it.

Experts believe the fake Nina Ricci container took up to three months to produce and was probably sanctioned at the highest levels of the Russian state.

A lab known as 'Q-ski' after the research and development division of the British Secret Service in James Bond, made the bottle and 'one-way' applicator nozzle so it was impossible for novichok to leak out in transit.

It was also made of special toughened glass, plastic or ceramic sure not to smash, crack or degrade while carrying one of the world deadliest weapons.

Rumours have also circulated that police are seeking up to four more Russian spies who were part of unit known as The Cleaners.

The attempted assassins are said to have had a back-up team who travelled separately and helped with reconnaissance in Salisbury, the Mirror reported.

It has previously been speculated that the perpetrators of the 2014 downing of Malaysian Airlines flight MH17 over eastern Ukraine, an act which killed 298 people, could have been killed to remove those who could one day testify about Moscow's role in the horror.

Piontkovsky said that after Dmitry Kovtun (left) Andrey Lugovoi were accused of poisoning Alexander Litvinenko they went public to protect themselves - but so far nobody has seen or heard from Petrov or Borishov

The men are accused of using a modified perfume bottle to spray deadly nerve agent on the front door of Sergei Skripal's home in Salisbury (pictured being decontaminated)

Sergei and daughter Yulia (pictured) collapsed and were rushed to hospital after the attack in March this year but survived

Meanwhile Russian state TV launched into fresh criticisms of Britain over the poisoning, amid protests over Putin's attempt to reform Russian pensions.

Vesti Nedeli's show accused London of an 'epidemic of lies', with presenter Dmitry Kiselyov - dubbed Putin's propagandist-in-chief, claiming May was using the Skripal case to cover-up the 'failure' of her Brexit policy.

TV shows slammed the idea that professional Russian agents would be so easily seen on CCTV images or fail to kill their target.

Itogi Nedeli's TV anchor Irada Zeynalova claimed the British prime minister needed a 'foreign enemy' to rally Britain, and chose Russia.

Petrov and Boshirov were being turned into Russian villains to the extent it was surprising they were not pictured in traditional Ushanka fur hats, she claimed.

Russian state-run TV assured its viewers that the novichok poisoning was the work of British or western secret services.

Weekly Vremya claimed the plot 'could not get any more absurd' and alleged Britain had invented crude stereotypes of Russian baddies as seen in Arnold Schwarzenegger film Red Heat.

The events showed the 'infantilism of British society'.

This was echoed by Russia's representative at the UN, Vasily Nebenzya, who claimed Britain had experienced a 'degradation' of its 'political culture'.

He said: 'It is difficult to imagine a serious court, even in the UK, for example, agreeing to examine the arguments of the British side.

'No one needs the truth, as a new, absolutely unique and efficient political ploy has been introduced ― lodging charges and pointing at the guilty without any proof.

'No one needs [to know who is] guilty.

'Or, rather, the guilty persons have been announced, but no one needs the justice.'

They had 'appointed' the Russian pair as being responsible for the Salisbury crime.