The Czech Republic on Saturday expelled the diplomats after saying that two alleged Russian spies accused of a nerve agent poisoning in Britain in 2018 were behind a deadly explosion at a Czech ammunition depot four years earlier.
The foreign ministry in Warsaw wrote on Twitter: "Allied solidarity and immediate reaction - this is what makes us strong. Poland fully supports the Czech Republic's decision to expel 18 Russian diplomats connected to the explosion of the… ammunition depot."
Russia on Sunday ejected 20 Czech diplomats in retaliation for Prague's move, the RIA news agency quoted the foreign ministry in Moscow as saying.
The row between Prague and Moscow is the biggest since the end of decades of Soviet domination of eastern Europe in 1989. It also adds to growing tensions between Russia and the West in general, raised in part by Russia's military build-up on its Western borders, the Reuters news agency reported.
Czech police said they were looking for two men who travelled to the Czech Republic days before the 2014 blast under the names Alexander Petrov and Ruslan Boshirov, according to Reuters.
Those names were the aliases used by the two Russian GRU military intelligence officers wanted by Britain for the poisoning of former Russian spy Sergei Skripal and his daughter with the Soviet-era nerve agent Novichok in the English city of Salisbury in 2018, Reuters added.