Russia has been researching the application of chemical agents to door handles as a way to assassinate its enemies, and has been training personnel “from special units” to carry out such attacks, said Mark Sedwill, Britain’s national security adviser, on Friday in a letter to the secretary general of NATO.

Mr. Sedwill’s letter, the most detailed account of British intelligence on the subject to be shared with the public to date, also reported that President Vladimir V. Putin of Russia was “closely involved in the chemical weapons program” beginning in the mid-2000s.

During that period, the letter claims, Russia was secretly developing the nerve agents known as Novichok that British officials say were used in the March 4 attack on Sergei V. Skripal and his daughter, Yulia, in the quiet cathedral city of Salisbury, England.

Russian officials have strenuously denied producing Novichok or carrying out the attack, which has brought relations between Britain and Russia to a post-Cold War low.