Czech police are hunting the two Russians wanted over the 2018 Salisbury poisonings in the UK in connection with a massive explosion at a Czech arms depot in 2014.
The country expelled 18 Russian embassy staffers on Saturday, who were accused of being intelligence operatives.
Detectives said they had ascertained Russia's military intelligence agency, the GRU, had been involved in the deadly blast in October 2014 in the eastern town of Vrbetice, in which two workers were killed.
They also accused the same two men wanted by the UK since September 2018 for the Salisbury poisonings, who have since been identified as GRU agents Anatoly Chepiga and Alexander Mishkin.
Czech police said the pair had travelled to the Czech Republic in October 2014 under the same cover identities, Alexander Petrov and Ruslan Boshirov, as they used in the UK in 2018.
Both men are understood to have been in the Czech Republic from October 11, 2014 until October 16, the day of the explosion. They were first in Prague and later in the eastern regions, where the depot was based.
Prime Minister Andrej Babis said his country could not let the revelations tying the blast to the GRU go unanswered, calling the circumstances "unprecedented and scandalous".
Russia orders mass expulsion of Czech diplomats in tit-for-tat move
Late on Sunday Russia ordered 20 Czech diplomats to leave the country in response to its 18 embassy staffers being expelled from the Czech Republic.
Czech Ambassador Vitezslav Pivonka was summoned to the Russian Foreign Ministry on Sunday evening and told that the 20 diplomats must leave by close of Monday.
Earlier Sunday, the Russian Foreign Ministry had called the expulsion of the Russians a “hostile step", adding: "In an effort to please the United States against the backdrop of recent American sanctions against Russia, the Czech authorities have even surpassed their overseas masters.”
“We are expressing our firm protest to the Czech authorities,” the statement added. “Our response will force the people behind these provocations to feel the full responsibility for their departure from the foundations of normal relations between our countries.”
Outraged protesters gather in Prague
Demonstrators gathered outside the Russian embassy in Prague on Sunday to condemn Russia and Russian president Vladimir Putin in the wake of the revelations from Czech police.
The largely peaceful demonstration saw protesters chant and wave placards aloft while riot police looked on.
Hundreds of people had to be evacuated from nearby villages after the explosion in Vrbetice on October 16, 2014, which claimed two lives.
Some 50 metric tons of ammunition had been stored at the site. Another explosion of 13 metric tons of ammunition occurred in the depot on Dec. 3 of that same year.
UK stands by Czech authorities
Czech Interior Minister Jan Hamacek said on Saturday that the 18 Russian embassy staffers had been "clearly identified" as spies from the Russian intelligence services, known as GRU and SVR.
They were ordered to leave the country within 48 hours.
Jakub Janda, the Director of the European Values Centre for Security Policy, has described Russia's alleged involvement in the 2014 explosion as "the largest Russian assault on the Czech territory since the 1968 invasion".
He called on the government to go further than expelling the 18 diplomats and to follow an example set by Britain in 1970s, in which more than 100 Soviet officials were expelled from the country over espionage.
The British foreign secretary, Dominic Raab, said on Sunday that the UK stood in "full support of our Czech allies", who he said had "exposed the lengths that the Russian intelligence services will go to in their attempts to conduct dangerous and malign operations in Europe".
Tom Tugendhat, chair of the UK parliament's foreign affairs select committee, also called for harsher measures to be taken against Russia by the UK.
He described the alleged Russian involvement in the Czech explosion as "warlike", adding: "We need to be much tougher about this. This is fundamentally about protecting the British people. And you can't protect the British people if you leave our allies exposed."