The UK has been encouraged by the “strength of support” from allies to take action against Russia after the nerve agent attack on a former spy and his daughter, Boris Johnson said just hours before the US secretary of state, Rex Tillerson, was sacked by Donald Trump.



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.

Play Video 0:49 Theresa May: highly likely Russia is behind Salisbury spy attack – video

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.”

Quick guide What is novichok? Show Hide Novichok refers to a group of nerve agents developed by the Soviet Union in the 1970s and 80s to elude international restrictions on chemical weapons. Like other nerve agents, they are organophosphate compounds, but the chemicals used to make them, and their final structures, are considered classified in the UK, the US and other countries. The most potent of the novichok substances are considered to be more lethal than VX, the most deadly of the familiar nerve agents, which include sarin, tabun and soman. Novichok agents work in a similar way, by massively over-stimulating muscles and glands. Treatment for novichok exposure would be the same as for other nerve agents, namely with atropine, diazepam and potentially drugs called oximes. The chemical structures of novichok agents were made public in 2008 by Vil Mirzayanov, a former Russian scientist living in the US, but the structures have never been publicly confirmed. It is thought they can be made in different forms, including as a dust aerosol. The novichoks are known as binary agents because they only become lethal after mixing two otherwise harmless components. According to Mirzayanov, they are 10 to 100 times more toxic than conventional nerve agents. Photograph: Matt Cardy/Getty Images Europe

May updated her cabinet on the unfolding investigation into the Salisbury attack, telling ministers there was “no doubt of the severity” of what had taken place.

Should Russia give no credible explanation, the UK is likely to expel a number of diplomats, more than the four who were told to leave the UK after the death of the former Russian security agent Alexander Litvinenko. The UK may also mount a covert cyber-offensive against the Kremlin.

Any direct action may be covered under article 51 of the UN charter, which allows legitimate self-defence, but Downing Street denied May was trying to win support to invoke article 5 of the Nato treaty on common defence.

The government may also decide to formally support amendments to the sanctions and anti-money-laundering bill to target the persecutors of Sergei Magnitsky, a Russian accountant who died after he revealed a major state-sponsored fraud.

Timeline Poisoned umbrellas and polonium: Russian-linked UK deaths Show Hide Georgi Markov In one of the most chilling episodes of the cold war, the Bulgarian dissident was poisoned with a specially adapted umbrella on Waterloo Bridge. As he waited for a bus, Markov felt a sharp prick in his leg. The opposition activist, who was an irritant to the communist government of Bulgaria, died three days later. A deadly pellet containing ricin was found in his skin. His unknown assassin is thought to have been from the secret services in Bulgaria. Alexander Litvinenko The fatal poisoning of the former FSB officer sparked an international incident. Litvinenko fell ill after drinking tea laced with radioactive polonium. He met his killers in a bar of the Millennium hotel in Mayfair. The pair were Andrei Lugovoi – a former KGB officer turned businessman, who is now a deputy in Russia’s state Duma – and Dmitry Kovtun, a childhood friend of Lugovoi’s from a Soviet military family. Putin denied all involvement and refused to extradite either of the killers. German Gorbuntsov The exiled Russian banker survived an attempt on his life as he got out of a cab in east London. He was shot four times with a silenced pistol. He had been involved in a bitter dispute with two former business partners. Alexander Perepilichnyy The businessman collapsed while running near his home in Surrey. Traces of a chemical that can be found in the poisonous plant gelsemium were later found in his stomach. Before his death, Perepilichnyy was helping a specialist investment firm uncover a $230m Russian money-laundering operation, a pre-inquest hearing was told. Hermitage Capital Management claimed that Perepilichnyy could have been deliberately killed for helping it uncover the scam involving Russian officials. He may have eaten a popular Russian dish containing the herb sorrel on the day of his death, which could have been poisoned. Boris Berezovsky The exiled billionaire was found hanged in an apparent suicide after he had spent more than decade waging a high-profile media battle against his one-time protege Putin. A coroner recorded an open verdict after hearing conflicting expert evidence about the way he died. A pathologist who conducted a postmortem examination on the businessman’s body said he could not rule out murder. Scot Young An associate of Berezovsky whom he helped to launder money, he was found impaled on railings after he fell from a fourth-floor flat in central London. A coroner ruled that there was insufficient evidence of suicide. But Young, who was sent to prison in January 2013 for repeatedly refusing to reveal his finances during a divorce row, told his partner he was going to jump out of the window moments before he was found.

Skripal poisoning Sergei Skripal and his daughter Yulia were were found unconscious on a bench in the Maltings shopping centre in Salisbury after 'suspected exposure to an unknown substance' which was later identified as chemical weapon novichok. In the aftermath Theresa May blamed Vladimir Putin and expelled 23 Russian diplomats who were suspected of spying. Two Russian men using the identities Ruslan Boshirov and Alexander Petrov were named as suspects. They appeared on Russian TV to protest their innocence.

The Skripals survived. However a local woman, Dawn Sturgess, died after spraying novichok on her wrists from a fake Nina Ricci perfume bottle converted into a dispenser, which had been recovered from a skip by her partner Charlie Rowley.

The measures, which the Foreign Office has previously resisted, saying it already has the full confiscatory powers it needs, are highly symbolic, having been already adopted by the US, but ministers are likely to support a specific “Magnitsky clause” in the bill at report stage.

Ministers may also consider whether to direct the broadcasting regulator, Ofcom, to investigate whether Russian media outlets, such as RT, are fit to hold broadcasting licences and whether senior officials could be told to boycott the football World Cup in Russia this summer.

Russia has denied being behind the attack on Skripal, a former double agent who came to the UK in a spy swap in 2010, and his daughter.

During an election campaign trip, the Russian president, Vladimir Putin, dismissed questions about the Skripals when confronted by the BBC, saying: “Get to the bottom of things there, then we’ll discuss this.”

The UK’s national security council was expected to meet on Wednesday to discuss the Russian response before the prime minister updates MPs. The home secretary, Amber Rudd, chaired a meeting of the government’s Cobra emergency committee in Whitehall on Tuesday morning to discuss the latest developments.