A retired Russian colonel and former double agent for MI6 is in a critical condition in hospital after being exposed to an "unknown substance" in Wiltshire.
Sergei Skripal, 66, and an as-yet unidentified woman said to be in her thirties, were found unconscious on a bench at the The Maltings shopping centre in Salisbury.
But who is Mr Skripal? And why may he still be of interest in Russia?
Mr Skripal was a Russian military intelligence officer given refuge in the UK after being jailed in Moscow for treason. He came to the UK as part of a high-profile “spy swap” in 2010 in which four men were exchanged for ten Russian "sleeper agents" in the US.
The double agent worked in GRU, Russian military intelligence, until at least 1999, when he officially transferred to the Ministry of Foreign Affairs. He worked there until 2003, when he left to pursue career in business. His arrest came a year later. In 2006, he was jailed for 13 years after admitting selling the names, addresses and code-names of "several dozen" Russian agents to MI6 over a period of ten years.
Mr Skripal had been recruited by MI6 and his motives for switching allegiance appear to have been financial.
In a country where the average wage was £160 per week, Mr Skripal received more than £54,000 to hand over information to Britain.
He was paid in cash each time he met his MI6 handler, while also receiving payments in a Spanish bank account.
Mr Skripal provided details which included the dates and locations of Russian agents' meetings with their Kremlin handlers while on trips to the UK and elsewhere in Europe – tip offs that MI6 used to ensure agents were placed under surveillance.
His betrayal was a serious blow to his birth country who said he had “caused serious damage to state security and to Russia's ability to defend itself".
Mr Skripal's is currently in hospital in Salisbury, in the southern English county of Wiltshire.
His wife died in 2012 and was buried in the UK. His son died last year in Russia while on holiday and was repatriated and buried in the UK.
The only member of Mr Skripal's family who was still alive at the time of the incident was his daughter.
The words of Victor Makarov, a former KGB agent who also turned and spied for Britain and once colleague of Mr Skripal, have an eerie sense of inevitability about them.
Like Mr Skripal, Mr Makarov defected to the UK after serving as a double agent and in 2007 from his house in northern England, Mr Makarov spoke of his fears that the KGB would seek revenge.
"They will try to shoot me in the back of the head, but they might use poison," he said.
"They never forget. When I was at the KGB in the 1970s they were still chasing people who had betrayed them 30 years before."