The Czech foreign ministry said an extensive investigation of the attack "led to a high degree of certainty about the responsible actor", naming it as China-linked group APT31.

"I summoned the Chinese ambassador to make clear that such hostile actions have serious consequences for our bilateral relations," Foreign Minister Jan Lipavsky said on X.

The foreign ministry of the Czech Republic, an EU and NATO member of 10.9 million people, said in a statement the attack started in 2022 and targeted "one of the unclassified networks" of the ministry.

"The malicious activity... was perpetrated by the cyberespionage actor APT31 that is publicly associated with the (Chinese) Ministry of State Security," the ministry added, citing its investigation.

"We call on the People's Republic of China to... refrain from such attacks and to take all appropriate measures to address this situation," said the ministry.

Lipavsky said that "we detected the attackers during the intrusion".

The Czech Security Information Office (BIS) singled out China as a threat to security in its 2024 annual report.

"The Chinese embassy logically focuses on gaining information about the Czech political scene," the BIS said.

EU foreign policy chief Kaja Kallas condemned the cyberattack in a statement.

"In 2021, we urged Chinese authorities to take action against malicious cyber activities undertaken from their territory," Kallas said, adding EU members have nonetheless witnessed attacks from China since then.

"We call upon all states, including China, to refrain from such behaviour, to respect international law and to adhere to the UN norms and principles, including those related to critical infrastructure," she added.

- 'Growing pattern' -

NATO also slammed the attack, saying it observed "with increasing concern the growing pattern of malicious cyber activities stemming from the People's Republic of China".

Prague has recently angered Beijing by fostering close ties with Taiwan as high-profile Czech delegations, including the parliament speakers, have visited the island while Taiwanese officials came to Prague several times.

China is trying to keep Taipei isolated on the world stage and prevents any sign of international legitimacy for the island.

It sees such visits as an infringement of the one-China policy which Prague officially pursues, just like the rest of the EU.

In May 2024, Lipavsky summoned the Russian ambassador over repeated cyberattacks targeting several European countries, including the Czech Republic, Germany and Poland.

They blamed the attacks on the Russian group APT28, also known as Fancy Bear, which has ties to Russia's GRU military intelligence service.

The BIS then said that Russia was a "permanent security threat" for the Czech Republic, which provides substantial humanitarian and military aid to Ukraine, battling a Russian invasion since 2022.

It added the Chinese threat was also growing in the context of the Ukraine war as "the North Korea-China axis keeps cultivating relations with Russia which give it a boost in the current conflict".

Beijing flatly denies allegations that it engages in state-organised hacking of overseas targets.