A British man poisoned along with his partner with a nerve agent, amid an assassination attempt on a Russian ex-spy in England blamed on Moscow, met its top UK envoy on Saturday, according to reports.

The Kremlin on Friday said it was "absurd" for Britain to dismiss as unconvincing an interview with two suspects in the poisoning of former double agent Sergei Skripal on British soil.

The leaders of Britain, the United States, France, Germany and Canada said on Thursday they had "full confidence" that the Novichok attack suspects were officers from Russia's military intelligence service.

The Kremlin denied on Thursday that Russia had been in any way involved in the poisoning of a former Russian spy and his daughter, describing British accusations that an attack had been approved by senior Russian officials as "unacceptable".

Judge Azmi Ariffin said the evidence presented in court since the trial started in October pointed to a "well-planned conspiracy" with a group of North Korean suspects who are still at large.

British counter-terrorism police said on Friday they believed they had found the source of the Novichok nerve agent that killed a woman in southwest England this month and left her partner Charlie Rowley critically ill in hospital.

Charlie Rowley's condition had improve overnight and was now "serious, but stable".

A British man poisoned with the Novichok nerve agent 10 days ago has regained consciousness and is now in a critical but stable condition, Salisbury District Hospital said on Tuesday.

Dawn Sturgess and Charlie Rowley, 45, fell ill last weekend in the town of Amesbury, near Salisbury.

Police on Friday raced to find the object that contaminated a British couple with the Soviet-made Novichok nerve agent in southwestern England where a former Russian spy was poisoned with the same toxin four months ago.

Britain is holding an emergency cabinet meeting on Thursday over a couple who were left critically ill after being exposed to the same nerve agent used on a former Russian spy earlier this year.

Skripal, 66, a former colonel in Russia's military intelligence who betrayed dozens of agents to Britain, and his daughter Yulia were found unconscious on a public bench in the southern English city of Salisbury on March 4.