Russia is the only nation with the "technical means, operational experience and motive" for the Salisbury nerve agent attack on a former spy and his daughter, Britain has said.
While Russia's London ambassador Alexander Yakovenko said the UK has not produced any evidence to support its claims about the incident, here are five reasons the UK thinks it highly likely Russia was responsible.
1 - Russia has the capability
Novichoks, which have been identified as the nerve agent used in the poisoning of former double agent Sergei Skripal and his daughter Yulia, was developed by the Soviet Union in the 1980s, according to the UK's National Security Adviser Sir Mark Sedwill.
In a letter to Nato Secretary General Jens Stoltenberg today Sir Mark said Russia continued its chemical weapons programme after the Soviet Union's collapse and described as "unlikely" any suggestion that Novichoks could be made and deployed by terrorist groups, especially at such high purity.
2 - Novichoks have been tested by the Russians on door handles
The highest concentrations of the nerve agent were discovered on the front door handle at Mr Skripal's house. Sir Mark said Russia had, in the 2000s, developed a programme investigating how to deliver Novichoks, which included applying them to door handles.
3 - Russia has a store of Novichoks
In the last 10 years Russia has made and stockpiled small quantities of the nerve agent under its chemical weapons programme, Sir Mark said.
4 - Russia has a record of assassinating so-called traitors
Russian president Vladimir Putin "probably" approved the killing of Alexander Litvinenko in 2006, a public inquiry in the UK concluded.
The dissident Russian spy died after drinking tea laced with radioactive polonium-210.
In the years since, there have been a number of suspected Russian state-sponsored assassinations, Sir Mark said.
5 - Sergei Skripal, as a former spy, may have been viewed as a legitimate target
Mr Skripal was convicted in his home country for selling secrets to the British and was freed in an exchange of captured spies between the US and Russia eight years ago.
He and his daughter had been spied on by Russian intelligence agencies for at least the past five years, Sir Mark said.
He said it is "highly likely" some defectors are still seen as "legitimate targets for assassination".