The Government has increased the pressure on Moscow to explain the circumstances around the poisoning of Kremlin critic Alexei Navalny by summoning the Russian ambassador to the UK to the Foreign Office over the suspected Novichok attack.

Foreign Secretary Dominic Raab said Britain was registering its “deep concern” about the alleged use of the nerve agent by summoning Andrei Kelin to speak with a senior official on Monday and called for Russia to carry out a “full, transparent investigation”.

Mr Raab has said it is “difficult to come up with a plausible alternative” to Moscow being behind the poisoning of the vocal critic of Russian President Vladimir Putin.

The Queen receiving Russian ambassador Andrei Kelin, during an audience at Buckingham Palace
The Queen receiving Russian ambassador Andrei Kelin, during an audience at Buckingham Palace (Jonathan Brady/PA)

The diplomat’s summoning came as Mr Navalny was brought out of a coma by doctors in Berlin, where the 44-year-old has been treated since falling ill on a domestic flight in Russia on August 20.

Mr Raab tweeted: “Today the UK summoned Russia’s Ambassador to the UK to register deep concern about the poisoning of Alexei @Navalny.

“It’s completely unacceptable that a banned chemical weapon has been used and Russia must hold a full, transparent investigation.”

Russia has denied that the Kremlin was involved in poisoning Mr Navalny.

But there is increasing pressure for Mr Putin to explain how his critic fell seriously ill, allegedly with the same chemical weapon used against Sergei Skripal, the Russian former double agent targeted in Wiltshire in 2018.

Berlin has threatened to rethink the fate of a German-Russian gas pipeline project if Moscow does not support an investigation, and Mr Raab has been in contact with his German counterpart Heiko Maas.

After the ambassador attended the Foreign Office, the department said: “There is a case here for Russia to answer.

“This took place on Russian soil, against a Russian citizen. They have international obligations to uphold. This is nothing short of an attack against the rules based international system which keeps our societies safe.”

Mr Kelin stressed during the meeting that voicing “unsubstantiated accusations” was “unacceptable”, according to the Russian embassy in London.

In response to the Skripal attack, Boris Johnson, as foreign secretary, helped corral a wave of expulsions of Russian diplomats across the EU and US after Britain told 23 envoys to leave.

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

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

Mr Navalny has been making a gradual recovery and the German hospital where he is being treated said he has been weaned off mechanical ventilation.