The Russian Embassy in Dublin has hit out at the way news of the alleged poisoning of a Russian man and his daughter in England has been "presented by the British authorities."

Sergei Skripal and his daughter Yulia remain in a critical condition in hospital in Salisbury after they were found slumped on a bench outside a shopping centre.

The Kremlin has denied suggestions Russia poisoned the 66-year-old - who was convicted in 2006 of high treason in Russia for his role as a double agent spying for MI6.

At the time is was alleged that he had disclosed the names of several dozen Russian agents working in Europe to the British spy agency.

He served a prison term in Russia before being handed over to US authorities and making his way to the UK.

He was handed over as part of a spy swap that saw 10 Russian "sleepers" who had been arrested in America exchanged for alleged double agents.

The current investigation is being led by the Counter Terrorism Unit, but is not being treated as terrorism.

Russia World Cup

Foreign Secretary Boris Johnson arrives at 10 Downing Street in London, for a meeting of the Government's COBRA emergency committee, 07-03-2018. Image: Dominic Lipinski/PA Wire/PA Images

Yesterday meanwhile, the British foreign secretary Boris Johnson went as far as to suggest England may boycott this summer's World Cup if there is shown to be Russian state responsibility in the alleged poisonings.

In a statement this afternoon the Russian Embassy took aim at the coverage the story has received over the past few days.

It claimed the story as it is being presented is "so convoluted that it requires really a close look" adding that "it seems to us that the British officials have very much explaining to do."

"One might question the motives of the British Foreign Secretary Boris Johnson, who has awkwardly put the blame for the attempt squarely on the Russian government without even waiting for the police protocol," it said.

COBRA

Meanwhile, British police have uncovered more information about the substance that left a former spy and his daughter in intensive care.

The British Home Secretary Amber Rudd disclosed the breakthrough at a COBRA meeting she chaired this morning.

A further statement from the London Metropolitan Police is expected this afternoon.

After the meeting this morning, Ms Rudd said: "Our thoughts are with everyone, especially the two people in hospital."

"This is likely to be a lengthy investigation. We need to make sure we respond to evidence, not rumour."

"We do know more about this substance. We must let the police carry on this work, and they will share more information this afternoon.

"There is a lot of information about him (Mr Skripal) but I am not going to comment further about that."

She would not be drawn to confirm whether Mr Skripal is a British citizen.

Ms Rudd also said "all the action necessary" had been taken to ensure the public was completely safe.

Mysterious illness

On Wednesday, London Met Police Assistant Commissioner Mark Rowley, the head of Counter Terrorism Policing, said: "The focus at this time is to establish what has caused these people to become critically ill."

"We would like to reassure members of the public that this incident is being taken extremely seriously and we currently do not believe there is any risk to the wider public," he said.

"The two people taken ill were in Salisbury centre from around 1.30pm.

"Did you see anything out of the ordinary? It may be that at the time, nothing appeared out of place or untoward but with what you now know, you remember something that might be of significance.

"Your memory of that afternoon and your movements alone could help us with missing pieces of the investigation. The weather was poor that day so there were not as many people out and about. Every statement we can take is important."

Cold relations

The incident is putting more strain on the already tense UK-Russia relationship, and Russia has denied any involvement.

In a press conference in Russia, Maria Zakharova of the foreign ministry said the link was being made to whip up anti-Russia sentiment. She would not confirm if Mr Skripal was still a Russian citizen.

The head of the Russia institute at King's College London said the Kremlin would be able to maintain plausible deniability and always shift suspicions to "innate Russophobia."

Dr Samuel Greene said: "The reality is we don't know the cause of the illness, so there's nothing to trace, we don't know where it's going to be traced to...

"Even if in fact this has been somehow sanctioned or ordered out of the Kremlin, the reality is that... the investigation is never going to find hard and fast evidence that would lead all the way up the chain of command.

"The Kremlin will in any case have tried to maintain some distance from this, and some plausible deniability."

Natasha Kuhrt, an expert on Russian foreign policy, said it was possible to surmise that Mr Skripal had made many enemies in his time as a double agent.

She said it was unlikely that billionaire Russians who had strong ties with Britain, living and sending their children to school here, would stop doing so, but that sanctions preventing them could have an impact.

Additional reporting from IRN ...