THE Kremlin has denied a claim that poisoned former Russian double agent Sergei Skripal wrote to Vladimir Putin asking to be pardoned and to be able to visit his home country.

The former Russian intelligence officer, who came to Britain in 2010 as part of a spy swap, regretted being a double agent and wanted to visit his family, his friend Vladimir Timoshkov told the BBC.

Skripal and his daughter Yulia remain in a critical condition after they were poisoned with the highly lethal nerve agent Novichok in Salisbury on March 4.

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Moscow has stepped up its campaign to discredit Theresa May's assertion that it is "highly likely" Russia was responsible for the attack.

The diplomatic crisis has plunged relations between Russia and the UK into the deep freeze and could trigger further action by European Union members in support of the Prime Minister's stance.

But the Russian embassy in London kept up its attempts to challenge the UK's case - reiterating its suggestion that the Porton Down defence laboratory was developing its own "military-grade poisons".

The lab, located less than 10 miles from Salisbury, is where tests were carried out to identify the Novichok substance.

Porton Down chief executive Gary Aitkenhead told the BBC: "We have got the highest levels of controls, of security around the work that we do here.

"We would not be allowed to operate if we had lack of control that could result in anything leaving the four walls of our facility here."

A Russian embassy spokesman said Aitkenhead's comment "amounts to admitting that the secret facility is a place where new components of military-grade poisons are being researched and developed".

On Friday, EU leaders promised an "unprecedented" diplomatic response to the attack after backing Theresa May's assertion that Moscow was responsible.

The bloc is recalling its ambassador to Russia for "consultations" on the Salisbury attack.

Russia has vehemently denied any responsibility for the incident, while on Sunday Vladimir Chizhov, Russia's EU ambassador, said that "from the legal point of view the Russian state had nothing against him [Skripal]".

The 66-year-old was accused of working for MI6 over several years, in particular disclosing the names of several dozen Russian agents working in Europe.

He was sentenced to 13 years in a high-security prison in August 2006, before being freed in the 2010 deal which saw 10 Russian sleeper agents expelled from the United States.

According to Timoshkov, his friend, who he had known since school, did not see himself as a traitor as he had sworn an oath to the Soviet Union.

"Many people shunned him. His classmates felt he had betrayed the Motherland," he said.

"In 2012 he called me. We spoke for about half an hour. He called me from London. He denied he was a traitor... (he told me) he wrote to Vladimir Putin asking to be fully pardoned and to be allowed to visit Russia. His mother, brother and other relatives were (in Russia)."

Russia's ambassador to the UK sent his well wishes to Skripal and his daughter on Friday, saying he hoped for their recovery.

Alexander Yakovenko wrote to Wiltshire Police detective sergeant Nick Bailey, who was left seriously ill after he was exposed to the poison as he went to the Skripals' aid.

Bailey said his experience had been "completely surreal" after he was discharged from hospital.