A hoax is likely to be one line of inquiry after a couple apparently fell ill in a Salisbury restaurant on Sunday prompting a major incident, police sources have told the BBC.
Alex King, 42, was admitted to hospital with his wife, six months after a nerve agent attack in the same city.
Mr King is a convicted criminal and once hoaxed Prince Charles.
Police sources said until the man had been spoken to, "we can't rule anything in or out".
On Tuesday, Mr King's wife, Anna Shapiro, 30, told the Sun that the Russian state had poisoned her and her husband in Prezzo.
The paper reported it was "what security sources fear was a suspected rat poison attack".
Ms Shapiro is a model and an Israeli citizen of Russian heritage.
A spokesman for the Sun told the BBC: "Like any newspaper, we were keen to talk to those at the centre of the incident and give them the opportunity to share with the public their version of events."
The article has since been removed from the newspaper's website "for legal reasons".
No arrests have been made and Wiltshire Police said on Wednesday: "This is now a routine investigation.
"The woman from the restaurant has been interviewed once and is likely to be spoken to again. The next step is to speak to the man involved."
The BBC was unable to contact the couple.
Wiltshire Police earlier confirmed the pair were not exposed to any nerve agent and that it was not linking their illness with the recent poisonings in Salisbury and Amesbury.
"Given recent events in Salisbury, the reporting of an event requiring the evacuation of bars and restaurants by police officers in bio-hazard suits, and that requires tests for the presence of Novichok, is of obvious public interest," a spokesman for the Sun added.
Salisbury District Hospital said "both patients are now medically fit" and have been discharged from hospital.
It is understood that the couple also tested negative for any other poisons, including rat poison.
In 2006 Mr King carried out a "prank" on the Prince of Wales at a film premiere.
He inserted himself into an official line-up of stars being greeted by Prince Charles and was later interviewed by the media about his deception.
He claimed the "prank" was carried out as part of a £100,000 bet with his employer - the convicted fraudster Edward Davenport.
In 2004, Mr King pleaded guilty at London's Horseferry Road Magistrates' Court to three counts of distributing indecent photographs or pseudo-photographs of children.
In a statement on Wednesday, Lorna Wilkinson, director of nursing at Salisbury District Hospital, said "given recent events" they had tested Mr King and Ms Shapiro for nerve agents, when they were admitted on Sunday evening.
"Tests on both patients came back negative," she said. "Both patients are now medically fit and there is no need for them to be in hospital."
"While this incident did not involve nerve agents, it was still clearly very stressful for our staff who, once again, demonstrated the very best of the NHS.
"I want to thank them for rising to the occasion yet again," she added.
On 4 March, Russian ex-spy Sergei Skripal and his daughter Yulia collapsed in Salisbury having been exposed to the nerve agent Novichok.
After weeks in hospital they were released, but in June, two Amesbury residents fell ill after being exposed to the same nerve agent. Dawn Sturgess, 44, later died.
The UK government has accused two Russian men, Alexander Petrov and Ruslan Boshirov, of attempting to kill the Skripals.
But in an interview on a Russian state-run news channel, they claimed to be tourists. Russia has denied any involvement in the poisoning.