Britain has demanded that Russia provide details about the Novichok nerve agent attack used on the Skripal family in March, after a man and a woman became critically ill on Saturday from the same poison.

The two – named by media as Dawn Sturgess, 44, and Charlie Rowley, 45 – handled the item near the site of the March attack on former double agent Sergei and his daughter Yulia in Salisbury, police say.

Russia has denied any involvement in the March incident and suggested the British security services had carried out that attack to stoke anti-Moscow hysteria.

“The eyes of the world are currently on Russia, not least because of the World Cup,” British Home Secretary Sajid Javid said on Thursday.

“It is now time that the Russian state comes forward and explains what has gone on.

Dawn Sturgess, 44, and Charlie Rowley, 45. Photo: Twitter

“It is completely unacceptable for our people to be either deliberate or accidental targets, or for our streets, our parks and towns to be dumping grounds for poison,” he told parliament.

The Kremlin said Russia had offered Britain its assistance in investigating the nerve agent attack and had been rebuffed.

Prime Minister Theresa May, speaking alongside Chancellor Angela Merkel during a visit to Berlin, said it was “deeply disturbing” that two more people had been exposed to Novichok, adding that her thoughts were with the people of the area.

Paramedics were called on Saturday morning to a house in Amesbury after Ms Sturgess collapsed. They returned later in the day when Mr Rowley also fell ill.

The two Britons taken ill on Saturday were initially thought to have taken an overdose of heroin or crack cocaine.

But tests by the Porton Down military research centre showed they had been exposed to Novichok.

“It is unbelievable that I am here to talk about another Novichok nerve agent incident in our county,” local police chief Keir Pritchard told reporters.

“We’re working extremely hard to try to understand the circumstances, the chronology.”

Sergei Skripl and daughter Yulia endured a long road to recovery. Photo: AAP

Britain has notified the global chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons.

Police have said the Skripals were poisoned after Novichok was applied in a liquid form to the front door of Mr Skripal’s home in the city of Salisbury, 11km south of Amesbury where the current incident occurred.

Mr Skripal, a former colonel in Russian military intelligence who betrayed dozens of agents to Britain’s MI6 foreign spy service, and his daughter were found slumped unconscious on a park bench on March 4.

The latest pair however have nothing in their background to suggest a link to the world of espionage or to Russia.

Interior minister Javid said the substance that the two people were exposed to was the same variant of Novichok that struck down the Skripals.

However, officials said the working hypothesis was that the latest pair had been contaminated in a different location from areas visited by the former Russian agent and his daughter.

“The possibility that these two investigations might be linked is clearly a line of inquiry,” head of counter-terrorism police Neil Basu told reporters.

“It’s the same nerve agent. Whether we can ever tell if it’s the same batch will be up to scientists to determine.”

Health chiefs said the risk to the public was low, repeating their earlier advice that the public should wash their clothes and use cleansing wipes on personal items.

“We’re satisfied that if anyone was exposed to that level of nerve agent by now they would be showing symptoms,” Mr Basu said.

-AAP