Police are continuing to treat an incident in Salisbury that triggered a fresh nerve agent scare as not suspicious, despite one of those caught up in it claiming she and her partner were targeted by Moscow.

In a bizarre twist to the novichok saga, Russian-born Anna Shapiro told a British red-top newspaper that she and her husband were the couple who had been dining at the Prezzo restaurant on Sunday and who had fallen ill at the premises.

Shapiro, 30, an events organiser and model, told the Sun her father was a Russian general. She claimed Moscow wanted her dead because she had turned her back on her homeland. She said her 42-year-old husband was still being treated at Salisbury district hospital and was seriously ill. She had discharged herself.

Police sources reacted with deep scepticism to Shapiro’s account. It is understood that police do not yet fully understand how the couple apparently became ill but tests ruled out poisoning with novichok or any other sort of nerve agent and have not so far suggested that any crime was committed.

Counter-terrorism police who investigated the novichok poisonings of the former spy Sergei Skripal, his daughter, Yulia, and the Wiltshire couple Charlie Rowley and Dawn Sturgess, have not been brought in to look at the latest incident.

On Wednesday afternoon Salisbury District hospital confirmed that both Shapiro and the 42-year-old man had been discharged. A spokeswoman said tests on both came up negative for novichok, adding: “Both patients are now medically fit and there is no need for them to be in hospital.”

A major incident was declared on Sunday night but Wiltshire police were quickly able to rule out novichok. The concern that there was more to the incident was given weight when a witnesses said the woman who was taken ill was Russian. Amanda Worne described the woman – apparently Shapiro – as a “beautiful blonde girl” but added: “She looked slightly dishevelled. She wasn’t relaxed. She was a bit stumbly, she wasn’t in control.”

Worne said the woman went upstairs to the toilets. “She came down hysterical, saying, ‘oh my god, we need an ambulance. Someone help’.” Worne said it became clear that the woman was with a man who had suffered a fit.

The street was cordoned off and paramedics, some in protective clothing, treated the couple before they were taken to hospital.

On Tuesday a Wiltshire police spokesperson said: “Tests have confirmed that the two patients who fell ill in Prezzo restaurant in Salisbury on Sunday evening were not exposed to any kind of nerve agent. Following test results, at this stage this is not being treated as suspicious.

“Inquiries into what caused the pair to become unwell are still ongoing. However, due to the recent events, scientific tests were undertaken to establish if they had come into contact with novichok, or any other type of nerve agent. These tests have concluded that this was not the case. Therefore, at this stage, we are not linking their illness to the recent poisonings.”

The Sun quoted unnamed security sources as saying the pair could have been attacked with strychnine, prompting Prezzo to issue a statement denying that it used the poison, often used to control rodents, at its Salisbury branch.

A spokesman said: “Responding to a report published by the Sun this morning we wish to confirm that the chemical strychnine is not used in Prezzo in Salisbury. This has been verified through independent technical support. We have been informed that Wiltshire police are not linking the event on Sunday evening to the recent nerve agent poisonings in Salisbury and Amesbury and have made no suggestion that the illness was a result of anything present in our restaurant.

“Based on the information we have received this is an isolated incident. We can also confirm that none of our team suffered any related illness, and all are well and keen to return to duty when the restaurant re-opens.They dealt with the circumstances of Sunday evening in an admirable manner and our thanks go to each of them. At Prezzo our sole aim is to care for our customers and the events of Sunday evening have shocked us all.”

Forensics officers in masks were seen working in the restaurant on Wednesday.

The spokesperson added: “Our Salisbury branch remains closed, as a precautionary measure only, while the police complete tests into the cause of the illness. We await information from the police.”