The US Secretary of State has been sacked after publicly backing the UK finding that Russia is "highly likely" to have used a weapons-grade nerve agent on British soil.
President Donald Trump, who initially avoided blaming the Kremlin over the assassination attempt in Salisbury, announced on Twitter that Rex Tillerson will be replaced in the role by CIA director Mike Pompeo.
The Washington Post reported that the US leader made the decision on Friday.
The news came as the international community rallied round Britain after the first use of this kind of chemical weapon in Europe since the Second World War.
A huge police inquiry was launched after former Russian double agent Sergei Skripal, 66, and his daughter Yulia, 33, were found slumped on a bench in Salisbury, Wiltshire, on March 4. They remain in a critical condition.
Detective Sergeant Nick Bailey, who was part of the initial police response, is in a serious but stable condition.
The episode has left Britain's relations with Moscow, which were already under severe strain, at breaking point.
In an extraordinary speech on Monday, Prime Minister Theresa May set a deadline of midnight on Tuesday for the Russians to provide a credible explanation as to why deadly Novichok was used on UK soil.
Peter Wilson, the UK Permanent Representative to the Organisation for the Prohibition of Chemical Weapons, told fellow delegates: "I did not expect to have to brief this Council on the first offensive use of a nerve agent of any sort on European territory since World War Two.
"The stark conclusion is that it is highly likely that Russia, a fellow State Party to the Chemical Weapons Convention (CWC), is implicated in chemical weapons use, whether by failure to control its own materials or by design.
"This attempted murder, using a weapons-grade nerve agent in a British city, was not just a crime against the Skripals. It was an indiscriminate and reckless act against the UK, which put the lives of innocent civilians at risk."
Foreign Secretary Boris Johnson said if Russia failed to provide "a convincing explanation" the UK response would be announced on Wednesday.
He added: "Russia is a great country. It is a great pity therefore that the Russian regime seems to be moving in this dangerous and disruptive direction."
The Government has a variety of counter-measures including economic, financial, and diplomatic moves. There has also been speculation about a potential covert cyber offensive against Russia in retaliation.
Prime Minister Theresa May told the regular weekly meeting of Cabinet at 10 Downing Street that there was "no doubt of the severity of what had taken place in Salisbury, which was a reckless, indiscriminate and despicable act".
Mrs May confirmed that she will chair a meeting of the National Security Council on Wednesday to discuss the Russian response and will then inform the House of Commons of any measures to be taken.
France, Germany and Mr Tillerson all gave their backing to the UK after the Government confirmed it was "highly likely" Russia was behind the assassination attempt.
However, President Donald Trump's White House press secretary Sarah Huckabee Sanders stopped short of pointing the finger at Russia.
Russian foreign minister Sergei Lavrov insisted the country was not to blame, and asked for access to samples of the poison.
News agency Tass quoted Russian foreign ministry spokeswoman Maria Zakharova as saying: "It is a circus show in the British Parliament.
"The conclusion is obvious, it's another political information campaign, based on a provocation."
Mr Putin dismissed questions about the Skripals when he was confronted during an election campaign visit, telling the BBC: "Get to the bottom of things there, then we'll discuss this."