Former Russian double agent Sergei Skripal had written to Vladimir Putin asking if he could return to his home country before he was poisoned in Salisbury, a friend has said.

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 be pardoned so he could visit his family in Russia, Vladimir Timoshkov told the BBC.

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

The 66-year-old had been 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 Mr Timoshkov, his friend, who he had known since school, Mr Skripal 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].”

The Kremlin later issued a denial that any such letter was sent to the Russian President.

The attempted assassination sparked a diplomatic crisis between Russia and Britain, which has been supported by its allies in apportioning blame to the Kremlin.

At least six countries in Europe are understood to be considering the expulsion of Russian spies after the EU took the lead and announced it would recall its ambassador to Moscow.

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Russia has vehemently denied any responsibility for the incident, while last Sunday the country’s EU ambassador Vladimir Chizhov said that “from the legal point of view the Russian state had nothing against him [Mr Skripal]”.

Russia’s ambassador to the UK Alexander Yakovenko sent his well wishes to Mr Skripal and his daughter on Friday and 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.

Sergei Skripal was freed in a 2010 deal which saw 10 Russian sleeper agents expelled from the United States (AP)