World leaders including Prime Minister Boris Johnson have demanded answers from Moscow amid claims Kremlin critic Alexei Navalny was poisoned using Novichok.

Mr Johnson described the use of the chemical weapon – also used in the 2018 Salisbury poisonings – as “outrageous” and vowed to ensure “justice was done” by working with international allies.

Britain has long accused Russian operatives of using the Soviet-era poison on Sergei Skripal, the former double agent targeted in the 2018 attack in the Wiltshire city.

Mr Navalny, a politician and corruption investigator, fell ill on a flight back to Moscow from Siberia on August 20 before being transferred to Berlin. The 44-year-old remains on a ventilator in intensive care.

The Prime Minister said: “We have seen first-hand the deadly consequences of Novichok in the UK.

“The Russian government must now explain what happened to Mr Navalny – we will work with international partners to ensure justice is done.”

Mr Johnson was supported by his predecessor Theresa May, who was prime minister at the time of the Wiltshire attack and called the use of chemical weapons “illegal and barbaric”.

She said: “Deeply alarming that Novichok has again been used in an attempt to silence critics of the Russian state.

“The international community must ensure that those who undermine our rules-based system cannot act with impunity.”

German Chancellor Angela Merkel said Mr Navalny was the victim of “attempted murder by poisoning” and the aim was to “silence” the fierce opponent of Russian President Vladimir Putin.

Ms Merkel had earlier said that “there are very serious questions now that only the Russian government can answer, and must answer” after new tests detected the use of Novichok.

The United States’ National Security Council (NSC) called the attack “reprehensible”.

NSC spokesman John Ullyot added: “Russia has used the chemical nerve agent Novichok in the past.

“We will work with allies and the international community to hold those in Russia accountable, wherever the evidence leads, and restrict funds for their malign activities.

“The Russian people have a right to express their views peacefully without fear of retribution of any kind, and certainly not with chemical agents.”

Mr Skripal and daughter Yulia were two of five people exposed to the substance in Wiltshire in 2018, both spending weeks in hospital recovering.

But Dawn Sturgess, 44, of Amesbury, Wiltshire, died in July 2018 after coming into contact with a perfume bottle thought to originally contain the poison, while her partner Charlie Rowley also spent nearly three weeks in hospital.

Detective Sergeant Nick Bailey, of Wiltshire Police, was also exposed to the substance while responding to the 2018 attack and almost died.

Sharing the PM’s tweet on Wednesday, he added: “I have so much that I want to say about this tweet. But I can’t, and I won’t.”

Foreign Secretary Dominic Raab also said that Russia has “a clear case to answer” after the German government said the Kremlin critic was targeted with Novichok in an attempted murder.

Mr Raab added: “It is absolutely unacceptable that this banned chemical weapon has been used again, and once more we see violence directed against a leading Russian opposition figure.

“The Russian government has a clear case to answer. It must tell the truth about what happened to Mr Navalny.”

The UK’s renewed pressure on Russia comes amid heightened tensions between the two nations. Mr Raab had already accused Moscow of trying to interfere in the 2019 general election.

Britain charged two Russians accused of being agents of the GRU military intelligence service over the Salisbury attack, but Russia has refused to extradite the men.

UK authorities said the Novichok was smuggled into England in a counterfeit Nina Ricci perfume bottle before being applied to the front door of Mr Skripal’s home.