Boris Johnson has said the UK will help “ensure justice is done” after the suspected poisoning of Russian opposition leader Alexei Navalny.
The critic of Vladimir Putin collapsed last week while campaigning in Siberia and is currently in a coma in a German hospital.
Doctors said that tests indicate he was poisoned.
The Prime Minister tweeted: “The poisoning of Alexey @Navalny shocked the world. The UK stands in solidarity with him & his family.
“We need a full, transparent investigation into what happened.
"The perpetrators must be held accountable & the UK will join international efforts to ensure justice is done.”
Mr Navalny's allies claim the Kremlin is responsible for the illness of its most prominent critic.
Some have demanded an investigation into whether Russian President Vladimir Putin was involved.
But the Kremlin has rejected claims of involvement in the alleged attack.
"These accusations absolutely cannot be true and are rather an empty noise," Kremlin spokesman Dmitry Peskov told reporters.
Mr Peskov saw no grounds for launching a criminal investigation into Mr Navalny's condition, saying that it could have been triggered by a variety of causes, and determining one of them should come first.
"If a substance (that caused the condition) is found, and if it is determined that it is poisoning, then there will be a reason for an investigation," Mr Peskov said.
Mr Navalny, a politician and anti-corruption investigator who is one of Mr Putin's fiercest critics, fell ill on a flight back to Moscow from Siberia on Thursday and was taken to a hospital in the city of Omsk after the plane made an emergency landing.
Over the weekend he was transferred to the Charite hospital in Berlin, where doctors said they have found indications of "cholinesterase inhibitors" in his system.
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.
Charite said Mr Navalny had undergone extensive examination by a team of physicians and that "clinical findings indicate poisoning with a substance from the group of cholinesterase inhibitors".
Cholinesterase inhibitors are a broad range of substances that are found in several drugs, but also pesticides and nerve agents. Charite said the specific substance to which Mr Navalny was exposed is not yet known but that a further series of comprehensive tests had been started.
Mr Navalny's wife, Yulia Navalnaya, has been visiting her husband every day and made no comment to reporters as she arrived on Tuesday.
