Dominic Raab has urged Moscow to “tell the truth” about the poisoning of Alexei Navalny, saying it is “absolutely unacceptable” the Russian opposition leader was poisoned by the same nerve agent deployed in Salisbury.

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

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

German Chancellor Angela Merkel said Mr Navalny, who is undergoing treatment in Berlin, was the victim of an “attempted murder by poisoning” and the aim was to silence Mr Navalny, a fierce opponent of Russian President Vladimir Putin.

Mr Raab said he was “deeply concerned” that Mr Navalny had been poisoned by Novichok, which led to the death of 44-year-old Dawn Sturgess after she was exposed to the nerve agent in Salisbury.

“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,” he said in a statement.

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

“We will work closely with Germany, our allies and international partners to demonstrate that there are consequences for using banned chemical weapons anywhere in the world.”

Boris Johnson echoed his calls, and said it was “outrageous” the chemical weapon had been used again.

The Prime Minister said Russia “must now explain what happened” and pledged that the UK will work to “ensure justice is done”.

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.

Meanwhile Nato secretary general Jens Stoltenberg added: “This is shocking, and I utterly condemn it. The use of a military-grade nerve agent makes it even more urgent that the Russian authorities conduct a full and transparent investigation.

“Those responsible must be identified and held accountable. We will be consulting with Germany and all Allies on the implications of these findings.

“Nato regards any use of chemical weapons as a threat to international peace and security.”

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 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.

Police 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.

He was hospitalised with his daughter Yulia, but they made a recovery. However, months later the bottle was found by local man Charlie Rowley who was taken ill alongside Ms Sturgess.