The Prime Minister has confirmed the UK backs a “transparent investigation” into the suspected poisoning of Russian opposition leader Alexei Navalny.

Mr Navalny, a politician and corruption investigator who is one of Russian president Vladimir Putin’s fiercest critics, fell ill on a flight back to Moscow from Siberia on Thursday August 20, and was taken to a hospital in the city of Omsk after the plane made an emergency landing.

Mr Navalny, who is in a coma, has since been transferred to a German hospital where doctors said tests indicated that he was poisoned.

The Kremlin has rejected accusations of involvement in an alleged attack against the prominent figure.

Boris Johnson tweeted: “The poisoning of Alexey Navalny shocked the world. The UK stands in solidarity with him and his family.

“We need a full, transparent investigation into what happened.

“The perpetrators must be held accountable and the UK will join international efforts to ensure justice is done.”

The Conservative Party leader’s statement follows calls from a host of senior MPs demanding an international investigation.

Tom Tugendhat, Tory Foreign Affairs Committee chairman, Chris Bryant, Labour chairman of the All-Party Parliamentary Group on Russia, and other select committee chairs last week wrote to the Russian ambassador in support of Mr Navalny and asked for a “full and independent investigation led by international investigating officers” into the allegations he was poisoned.

German chancellor Angela Merkel has already demanded a probe take place into what happened to the Putin rival after doctors in the country discovered “cholinesterase inhibitors” in his system following his transfer over the weekend to the Charite hospital in Berlin.

Cholinesterase inhibitors act by blocking the breakdown of a key chemical in the body, acetycholine, that transmits signals between nerve cells. Mr Navalny is being treated with the antidote atropine.

Allies of the politician have claimed the Kremlin is behind the illness of its most prominent critic, with some demanding an investigation into whether Mr Putin was involved.

But Kremlin spokesman Dmitry Peskov told reporters this week: “These accusations absolutely cannot be true and are rather an empty noise.

“We do not intend to take it seriously.”

The developments follow the poisoning and murder on UK soil of former Russian spy Alexander Litvinenko in 2006, along with the Novichok attack on double agent Sergei Skripal and his daughter in Salisbury two years ago.

British investigations into both incidents concluded that Russian agents were behind the attacks.