A picture has emerged of what is believed to be a Russian double agent shortly before he fell critically ill at a shopping centre in Salisbury.

British counter-terror police are aiding a probe into what caused former MI6 spy Sergei Skripal and his daughter Yulia to fall ill at The Maltings shopping centre on Sunday afternoon, with officers said to be interested in the two people shown in the picture.

It was taken from a CCTV camera at 3.47pm on Sunday at Snap Fitness 24/7 gym and shows a man and a woman walking through an alleyway connecting a Zizzi restaurant and the bench where Mr Skripal and his daughter were found.

Gym manager Cain Prince said: "Police had a good look at the footage and were interested in these two people. It was the only image they took away. They wanted a list of everyone in the gym between 3pm and 4pm as well."

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The image emerged after Metropolitan Police assistant commissioner Mark Rowley confirmed that his specialists were supporting the investigation into what had caused Sergei and Yulia Skripal to fall ill.

The Kremlin has denied suggestions Russia poisoned Mr Skripal, who was convicted in the country of spying for MI6 in 2006. It was alleged that he disclosed the names of several dozen Russian agents working in Europe

When asked about speculation in the British press that Russia had been involved with Mr Skripal's illness, Kremlin spokesman Dmitry Peskov said: "It didn't take them long."

Mr Skripal's illness was "a tragic incident", he added.

The Kremlin has said Moscow is "always open for cooperation" regarding the British investigation, which Mr Rowley described as a "very unusual case".

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Image: Sergei Skripal was a colonel in Russian military inteligence

"The critical thing is to get to the bottom of what has caused this incident as quickly as possible," he told BBC Radio 4's Today programme.

"As you would expect, the specialist resources that sit within the counter-terrorism network that I coordinate across the country and other partners are working with Wiltshire Police to get to the bottom of that as quickly as possible.

"If you look back at other cases like (Alexander) Litvinenko, if necessary we will bring that investigation into the counter-terrorism network.

"At the moment the key is, though, to get to the bottom of what caused this."

2004: Skripal arrested by Russian security

Mr Skripal, a former colonel within Russian military intelligence, did not have any "visible injuries" when he was taken to hospital, Wiltshire Police's temporary assistant chief constable, Craig Holden, revealed.

The Zizzi restaurant in Castle Street close to where Mr Skripal was found was also closed "as a precaution" and a major incident was declared at Salisbury District Hospital, where Mr Skripal and his daughter remain in a critical condition.

Justice Secretary David Gauke declined to comment on the investigation on Tuesday, while the Russian Embassy in the UK claimed that "neither relatives nor legal representatives of the said person, nor the British authorities" had been in touch regarding the probe.

Business minister Claire Perry told Sky's Beth Rigby that the incident will be discussed at a cabinet meeting on Tuesday, where an update is expected from Home Secretary Amber Rudd.

Vienna 2010: The exchange of spies

Mr Skripal was sent to Britain as part of a spy swap in 2010, having been given a pardon from his prison sentence.

As part of the deal, 10 Russian sleeper agents were expelled from the United States, including Manhattan socialite and diplomat's daughter Anna Chapman, who was married to a British man and lived in London for several years.

The Salisbury incident has drawn comparisons with the death of Russian dissident Mr Litvinenko in 2006, who died in London after drinking tea laced with radioactive polonium-210.