Tillerson, who spoke to the foreign secretary on Monday afternoon, had told reporters the attack on Sergei and Yulia Skripal “clearly came from Russia” and would have consequences.
His remarks went further than those of Theresa May, who told the House of Commons on Monday it was “highly likely” Russia was behind the attack. The White House press secretary, Sarah Huckabee Sanders, had stopped short of pointing the finger at Russia.
Just hours after Johnson had welcomed US support, Trump tweeted that he had replaced Tillerson with the CIA director, Mike Pompeo. The sacking may not be linked to Tillerson’s comments on Russia; relations between the pair are believed to have been deteriorating for some time, especially over the Iran nuclear deal and Trump’s announcement that he would meet the North Korean leader, Kim Jong-un.
It is unclear when Tillerson learned that his dismissal was imminent. It was first reported he had known since Friday, which was denied by sources, and a state department spokesman later said Tillerson “did not speak to the president and is unaware of the reason” and later suggested he had read the news on Twitter.
The US president said he would speak to the British prime minister about the Salisbury poisoning on Tuesday.
Trump said: “It sounds to me like they believe it was Russia … I would certainly take that finding as fact.” But he added: “If we get the facts straight we will condemn Russia, or whoever it might be.”
Downing Street is hoping for a strong statement of support from Trump when he speaks with May on Tuesday, having previously been encouraged by such direct condemnation from Tillerson.
Skripal and his daughter remain in hospital in a critical condition while the Wiltshire police detective sergeant Nick Bailey is in a serious but stable condition.
Bailey is making good progress, Britain’s most senior counter-terrorism police officer said. Delivering an update on the police investigation outside New Scotland Yard on Tuesday, the Metropolitan police deputy assistant commissioner, Neil Basu, said 38 people were seen by medical staff in the aftermath of the “reckless, despicable and targeted” attack.
Of those, 34 have been assessed and discharged and one more person is still being monitored as an outpatient but is not showing signs of illness. Previously, police had said 21 people had been affected.
In an interview earlier on Tuesday, Johnson repeated May’s ultimatum to the Kremlin that it must explain by midnight on Tuesday if it was behind the attack, or if it had allowed the deadly nerve agent novichok to get into the wrong hands.
“If they can come up with a convincing explanation, then obviously we will want to see full disclosure of that to the Organisation for the Prohibition of Chemical Weapons in The Hague,” Johnson said.
The Russian foreign minister, Sergei Lavrov, said Russia had requested access to the substance to perform its own checks but the request had been refused. May’s spokesman hit back at Lavrov’s suggestion that Britain could be violating the chemical weapons convention.
“The UK fully complies with all of its obligations under the chemical weapons convention,” the spokesman said. “Under the chemical weapons convention, states have the mechanism to consult but there is no requirement to do so.”