Police officers seal off the road on which Russian Sergei Skripal and his daughter have been staying in Salisbury, Britain, March 7, 2018.

Russian former double agent Sergei Skripal and his daughter were poisoned with a military-grade nerve toxin that had been left on the front door of their home in England, British counter-terrorism police said.

After the first known offensive use of a chemical weapon on European soil since World War Two, Britain blamed Russian President Vladimir Putin for the attempted assassination and the West has expelled around 130 Russian diplomats.

Russia has denied using Novichok, a nerve agent first developed by the Soviet military, to attack Skripal, and Moscow has said it suspects the British secret services are trying to frame Russia to stoke anti-Russian hysteria.

"We believe the Skripals first came into contact with the nerve agent from their front door," said Dean Haydon, Britain's' senior national coordinator for counter-terrorism policing.

"Specialists have identified the highest concentration of the nerve agent, to date, as being on the front door of the address," Scotland Yard said in a statement.

Skripal and his 33-year-old daughter, Yulia, have been in a critical condition since being found unconscious on a public bench in the English city of Salisbury on March 4. A British judge has said they may have suffered permanent brain damage.

The attempted murder of Skripal, a 66-year-old former colonel in Russian military intelligence who betrayed dozens of Russian agents to Britain's MI6 spy service, has plunged Moscow's relations with the West to a new post-Cold War low.

After Britain expelled 23 Russians it said were spies working under diplomatic cover, Russia followed by throwing out 23 British diplomats. The United States and other Western countries, including most member states of the European Union and NATO, expelled over 100 diplomats.

British lawmakers launched a new inquiry into money-laundering, sanctions and economic crime on Thursday, with a particular focus on properties bought with so-called "dirty money".