Two Russian spies were arrested while allegedly on their way to a laboratory that tested novichok samples from Salisbury, it has emerged.

The two suspects, who are not the same pair charged with launching the attack on Sergei Skripal, were detained in the Hague in spring and sent back to Russia.

Intelligence services suspected they were on their way to the Spiez Laboratory in Switzerland, sources told the Tages-Anzeiger and NRC Handelsblad.

Accredited by the Organisation for the Prohibition of Chemical Weapons (OPCW), the institution has been charged with testing substances used in Salisbury, Amesbury and Syria.

A spokesperson for the Swiss Federal Intelligence Service (NDB) told The Independent that authorities “are aware of the case of Russian spies discovered in the Hague and expelled from the same place”.

“The Swiss Federal Intelligence Service (FIS) participated actively in this operation together with its Dutch and British partners,” she added. ”The FIS has thus contributed to the prevention of illegal actions against a critical [part of] Swiss infrastructure.”

Officials at the Spiez Laboratory said it had also been targeted with cyber attacks, including a fake conference invitation that contained malicious software.

The spies, who were allegedly found with espionage equipment that could be used to spy on the laboratory, were sent back to Russia and have not been prosecuted.

Inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) arrive to begin work at the scene of the nerve agent attack (Reuters/Peter Nicholls) (REUTERS/Peter Nicholls)

Spiez became the centre of conspiracy theories spread by Russian state media earlier this year, which falsely claimed that western-made BZ nerve agent and its precursors rather than novichok were found in the Skripal samples.

Foreign minister Sergei Lavrov used the claims to turn fire on Britain by accusing it of producing BZ and violating the chemical weapons convention.

Ahmet Uzumcu, the former head of the OPCW, later said a precursor chemical of BZ and a blank sample was sent to all designated laboratories alongside real evidence taken from Salisbury to prove their tests were accurate.

“The BZ samples did not have anything to do with the Salisbury samples,” he added in May. “It was solely for checking the quality of the work.”

Russia has been seeking to discredit OPCW’s verification of chemical weapons used in Salisbury and Syria, where it is supporting Bashar al-Assad’s forces.

The Kremlin has used its power of veto at the UN Security Council to block any resolutions against the Syrian government, while the Russian Embassy in London has claimed the OPCW “lacks transparency”.

Tensions with Britain have risen yet again after the government identified the two men accused of launching the Salisbury novichok attack as Russian spies from the GRU military intelligence agency.

On Friday, a Kremlin spokesman suggested that it would not allow British access to the suspects.

“We don’t organise interviews with citizens of Russia,” Dmitry Peskov said: “There are mechanisms to provide legal assistance... if the British decide to make an application, we will respond strictly according to law.”

Police release images of suspects in connection with Salisbury attack Show all 19 1 /19 Police release images of suspects in connection with Salisbury attack Police release images of suspects in connection with Salisbury attack CCTV – Salisbury train station The two suspects charged in relation to the attack on Sergei and Yulia Skripal at Salisbury train station at 16:11hrs on 03 March 2018 Metropolitan Police Police release images of suspects in connection with Salisbury attack Suspects Suspects Ruslan Boshirov and Alexander Petrov, Russian nationals, approximately 40 years old, who travelled on a Russian passport. It is likely that they were travelling under aliases and that these are not their real names Metropolitan Police Police release images of suspects in connection with Salisbury attack Evidence Bottle and applicator recovered by police from Charlie Rowley’s address in Muggleton Road Metropolitan Police Police release images of suspects in connection with Salisbury attack Evidence A counterfeit perfume box that was discovered by nerve agent victim Charlie Rowley, who later gave it, and the bottle inside, to his girlfriend Dawn Sturgess Metropolitan Police/AFP/Getty Police release images of suspects in connection with Salisbury attack Rowley has previously said he felt lucky to be alive after giving a perfume bottle that contained the nerve agent Novichok to his girlfriend Dawn Sturgess, who later died Metropolitan Police/AFP/Getty Police release images of suspects in connection with Salisbury attack Suspeccts The CPS has issued European Arrest Warrants for the extradition of 'Boshirov' and 'Petrov' in connection with the Novichok poisoning attack on Sergei Skripal and his daughter Yulia in March Metropolitan Police/PA Police release images of suspects in connection with Salisbury attack CCTV – ‘Boshirov' at Gatwick airport Movements in detail - At 3pm on Friday, 2 March, the suspects arrived at Gatwick airport, having flown from Moscow on Aeroflot flight SU2588 Metropolitan Police Police release images of suspects in connection with Salisbury attack CCTV – ‘Petrov’ at Gatwick airport From the airport it is believed that they travelled by train into London, arriving at Victoria station at approximately 5.40pm Metropolitan Police Police release images of suspects in connection with Salisbury attack CCTV – Salisbury train station, 11:48hrs on 4 March 2018 They then travelled on London public transport to Waterloo station and were in the area between approximately 6pm and 7pm. They travelled to the City Stay Hotel in Bow Road, East London, where they stayed on Friday, 2 March, and Saturday, 3 March. On Saturday, 3 March, they left the hotel and took the underground to Waterloo station, arriving at approximately 11.45am, where they caught a train to Salisbury, arriving at approximately 2.25pm Metropolitan Police Police release images of suspects in connection with Salisbury attack City Stay Hotel in Bow Road Police officers stand outside the City Stay Hotel in Bow where on Sunday, 4 March, 'Boshirov' and 'Petrov' made the same journey from the hotel as they did the previous day, again using the underground from Bow to Waterloo station at approximately 8.05am, before continuing their journey by train to Salisbury Getty Police release images of suspects in connection with Salisbury attack CCTV – Wilton Road, Salisbury, 11:58hrs on 4 March 2018 CCTV shows them in the vicinity of Mr Skripal’s house and we believe that they contaminated the front door with Novichok Metropolitan Police Police release images of suspects in connection with Salisbury attack CCTV – Fisherton Road CCTV image of both suspects on Fisherton Road, Salisbury at 13:05hrs on 4 March, 2018 Metropolitan Police Police release images of suspects in connection with Salisbury attack CCTV – Salisbury train station, 13:50hrs on 4 March 2018 They left Salisbury and returned to Waterloo Station, arriving at approximately 4.45pm and boarded the London Underground at approximately 6.30pm to London Heathrow Airport Metropolitan Police Police release images of suspects in connection with Salisbury attack CCTV – Heathrow airport security, 19:28hrs on 4 March 2018 From Heathrow Airport, they returned to Moscow on Aeroflot flight SU2585, departing at 10.30pm Metropolitan Police/PA Police release images of suspects in connection with Salisbury attack Scene investigations The police investigation was carried out over 6 months. Ex-Russian spy Sergei Skripal and his daughter Yulia were found on March 4 in a critical condition on a bench outside the Maltings shopping centre in Salisbury AFP Police release images of suspects in connection with Salisbury attack Victims Former Russian spy Sergei Skripal, right, and his daughter Yulia Rex Police release images of suspects in connection with Salisbury attack Victims Detective Sergeant Nick Bailey who rushed to the aid of the Skripals was also taken to hospital in a serious condition after falling ill when attempting to help them PA Police release images of suspects in connection with Salisbury attack Victims Dawn Sturgess, a 44-year-old mother of three, died after falling ill when partner Charlie Rowley gave her a perfume bottle that contained the nerve agent Novichok Facebook/AFP/Getty Police release images of suspects in connection with Salisbury attack Scene investigations The home of Charlie Rowley in Muggleton Road, Amesbury, where he and Dawn Sturgess were exposed to the deadly nerve agent Novichok PA

Police released images of the men last week and said they arrived in Britain two days before the poisoning on business visas and genuine Russian passports in the names of Alexander Petrov and Ruslan Boshirov.

Two men claiming to be the suspects appeared on the state-funded RT television network on Thursday after Vladimir Putin publicly called on them to give an interview.

They claimed the suspected aliases were their real names and said they visited Salisbury two days in a row to see its “famous” cathedral – even though CCTV showed them walking in the opposite direction towards Mr Skripal’s house on both days.

The men explained their extensive travel history around Europe, which is now the subject of close scrutiny, by claiming to be businessmen selling sport nutrition supplements.

The pair also denied they were carrying novichok, or were in possession of a specially adapted Nina Ricci perfume bottle UK police say was used to administer the poison.

Security minister Ben Wallace told the House of Commons that they smuggled the novichok into Gatwick airport using the counterfeit bottle, which was “recklessly” discarded after being used to smear the nerve agent on Mr Skripal’s front door and later poisoned Charlie Rowley and killed his partner Dawn Sturgess.

A Downing Street spokesman dismissed the suspects’ interview as “lies and blatant fabrications”, which were an “insult to the public’s intelligence”.

“More importantly, they are deeply offensive to the victims and loved ones of this horrific attack,” he added. “Sadly, it is what we have come to expect. An illegal chemical weapon has been used on the streets of this country.

“We have seen four people left seriously ill in hospital and an innocent woman has died. Russia has responded with contempt.”

Sajid Javid has vowed that Britain and its allies will catch the pair if they ever leave Russia again using European Arrest Warrants and Interpol red notices, but admitted the scenario is unlikely.