Boris Johnson has called for answers from the Kremlin over the poisoning of a Putin critic with novichok.
He added: "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."
The Kremlin was accused of poisoning former Russian spy Sergei Skripal and his daughter with novichok in Salisbury in 2018. Moscow has denied any involvement.
Britain's foreign minister said it is "absolutely unacceptable" a banned chemical weapon has been used again.
Dominic Raab said: "The Russian government has a clear case to answer. It must tell the truth about what happened to Mr Navalny.
More from Alexei 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."
Mr Navalny was transferred to a Berlin hospital after falling ill on an internal Russia flight on 20 August, with doctors in Omsk where he was in hospital saying he had pancreatitis and was not poisoned. He remains in a serious but stable condition in Berlin.
It’s outrageous that a chemical weapon was used against Alexey Navalny. 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.
— Boris Johnson (@BorisJohnson) September 2, 2020
Health Secretary Matt Hancock added that the UK will "stand ready to offer all the support that's available to help Germany, to investigate and take action as necessary".
Marina Litvinenko, the wife of Russian defector Alexander Litvinenko who died after being poisoned with radioactive polonium in 2006, said many people in Russia believed Mr Navalny was "on safe ground".
She told Sky News: "Now, it looks like this regime can do whatever they want.
"After what happened with my husband, everyone said 'Russia is not safe', but despite all the evidence Russia would not stop.
"It's a big question for Western governments, do you really understand who you are dealing with in Vladimir Putin? People may be killed, may be poisoned and after that will you do business as usual or make some kind of decision?"
She believes the Kremlin allowed Mr Navalny to be transferred to a German hospital because whoever poisoned him did not think novichok would be detected.
Mrs Litvinenko added that in her husband's case, the UK was the only country to stand up to Russia, and said the entire international community needs to do so now if Russia is to be held properly accountable.
Labour's shadow minister for Europe said the development is "serious and alarming" and said the UK must support the referral of Mr Navalny's poisoning to the Organisation for the Prevention of Chemical Weapons (OPCW).
A White House spokesman called Mr Navalny's poisoning "completely reprehensible" and said the US will work with the international community to hold those in Russia accountable.
The US will also restrict funds for their "malign activities", he added.
German chancellor Angela Merkel said Mr Navalny's poisoning was attempted murder and called for Russia to carry out an investigation into who poisoned him, while also promising an investigation by the EU and NATO.
The Kremlin said it could not give a proper response yet and it has requested a full exchange of data between Berlin and Moscow, as well as full co-operation.
A source with details of a Russian inquiry into Mr Navalny's hospital admission said the investigation is "a purely procedural formality and there are still no reasons whatsoever to assume that Navalny was poisoned", according to Russian state news agency Interfax.
French foreign affairs minister Jean-Yves Le Drian said: "I wish to condemn in the strongest possible terms the
shocking and irresponsible use of such an agent."
NATO secretary general Jens Stoltenberg said he "utterly" condemns the use of novichok and called on Russia to conduct a "full and transparent investigation", as he added that NATO will consult with Germany and other allies on the implications.
Ursula von der Leyen, president of the EU Commission, said Mr Navalny's poisoning "is a despicable and cowardly act – once again".
"Perpetrators need to be brought to justice," she added.
Lithuania's foreign affairs minister also condemned the poisoning, saying "you can't buy this in drug store" as he referred to novichok.
Linas Linkevicius tweeted those responsible "must face consequences" and "not too many lines remain uncrossed".