British police believe they have identified the suspects who carried out the Novichok nerve agent attack on a former Moscow double agent and his daughter and that they are Russian, the Press Association news agency reported Thursday.

"Investigators believe they have identified the suspected perpetrators of the Novichok attack through CCTV and have cross-checked this with records of people who entered the country around that time," a source with knowledge of the probe told the British agency.

"They (investigators) are sure they (suspects) are Russian," the source added.

Government departments and the Metropolitan Police, whose counter-terrorism unit is investigating the poisoning, refused to comment on the report when contacted by AFP.

But Security Minister Ben Wallace wrote on Twitter: "I think this story belongs in the 'ill-informed and wild speculation folder'."

Russia's embassy in London seized on the minister's message and urged Britain "to conduct the investigation in a transparent way so as to avoid rumours and leaks".

"In March British political leadership rushed to put forward unsubstantiated accusations against Russia, instead of waiting for results of the investigation," it added in a statement.

Ex-Russian spy Sergei Skripal and his daughter Yulia collapsed in the southwestern English city of Salisbury after being exposed to the nerve agent Novichok.

Both have since recovered.

AAP

Britain blamed Russia for the poisoning of Skripal, a former military intelligence colonel who was jailed for betraying Russian agents to Britain's MI6 foreign intelligence service.

He left Russia for Britain in a 2010 spy swap.

Russia has strongly denied involvement in the Skripal attack, sparking a row that has led to tit-for-tat diplomatic expulsions between Britain and its allies and Moscow.

'Coded Russian message'

In another report Thursday, CNN said the two suspects were identified using facial recognition technology and had fled the country following the March 4 attack, likely on a commercial flight.

Britain previously confirmed that border officials searched an incoming Aeroflot plane at London's Heathrow Airport on March 31 in what it called "routine" checks for criminal activity.

Russia at the time called the search an act of "blatant provocation" and suggested it was linked to the diplomatic spat over Salisbury.

Citing a source with knowledge of the investigation, CNN said the suspects' departure was "revealed in a coded Russian message to Moscow sent after the attack, which was intercepted by a British base in Cyprus".

Two Britons fell ill last month in a town near Salisbury after being exposed to Novichok, one of whom died.

Experts are seeking to establish whether the toxin was from the same batch used against the Skripals.

Dawn Sturgess, 44, and her 45-year-old partner Charlie Rowley collapsed at his house in Amesbury within hours of each other on June 30.

AP

Sturgess died on July 8, prompting police to open a murder investigation, while Rowley has regained consciousness and is in stable condition.

The Press Association said investigators believe Sturgess was exposed to at least 10 times the amount of nerve agent as the Skripals came into contact with.

Police have said it was detected in a "small bottle" at Rowley's house.

Rowley's brother Matthew told the BBC that the 45-year-old had told him the Novichok was contained in a perfume bottle.

Fingertip search

The Organisation for the Prohibition of Chemical Weapons earlier this week said its inspectors had collected samples in the Amesbury case and were analysing them.

read more Russian agency suspected in US election hack may be behind UK poisoning

An inquest into the death of Sturgess opened on Thursday in Salisbury and was adjourned until a pre-inquest review on January 16, pending further investigation.

In English law, inquests are held to examine violent, unnatural or unexplained deaths. They set out to determine the place, time and type of death, but do not apportion blame.

On Wednesday, police started a search of the Queen Elizabeth Gardens in Salisbury. Certain areas were to be searched by fingertip, in a search due to last several days.

The New York Times reported Sunday that British investigators believe the Skripals were targeted by current or former agents of the GRU Russian military intelligence agency.

The GRU, where Skripal worked, is also accused of interfering with the 2016 US presidential election. Twelve of its officers were charged Friday with hacking Hillary Clinton's campaign.