Russian intelligence agents hacked Yulia Skripal's emails for at least five years before she and her father Sergei were poisoned in Salisbury, newly-declassified Government intelligence has revealed.

Cyber specialists from the GRU - Moscow's Main Intelligence Directorate - targeted email accounts belonging to Miss Skripal as long ago as 2013, and possibly even before that, according to the British security services, in an apparent attempt to track the Skripals’ movements.

Theresa May took the highly unusual decision to release previously classified intelligence in order to quash weeks of Russian “disinformation” about the source of the attack.

The dossier also says Russian agents tested the effectiveness of Novichok nerve agent smeared on door handles as part of a secret chemical weapons programme codenamed Foliant in which President Vladimir Putin was “closely involved”.

Police investigating the attempted murder of Yulia Skripal and her father Sergei have previously said that the highest concentrations of Novichok were found on Mr Skripal's front door, suggesting that was where the nerve agent had been deposited.

The details were revealed in a letter from Sir Mark Sedwill, Theresa May's National Security Adviser, to the Nato Secretary General Jens Stoltenberg which was released yesterday by Downing Street. It followed days of delicate negotiations between ministers and Britain’s intelligence chiefs about what could be put into the public domain.