Hackers have carried out a major attack on Germany's centre-right Christian Democratic Union party, the Interior Ministry confirmed on Saturday.
The ministry did not provide further information about the extent or nature of the attack, which is being investigated by Germany's domestic security service, the Office for the Protection of the Constitution.
"However, the nature of the attack points to a very professional actor," a spokesman said.
The incident - one week before elections to the European Parliament - is being taken very seriously, according to sources close to the government.
Interior Minister Nancy Faeser has reportedly been in contact with CDU party leader Friedrich Merz.
The CDU, the party of former chancellor Angela Merkel, said it was working closely with German security authorities and other external experts.
The Federal Office for the Protection of the Constitution is set to issue a warning to all parties in the German parliament on Saturday, the Interior Ministry spokesman said.
"Our security authorities have ramped up all protective measures against digital and hybrid threats and are providing information on the dangers. We are once again seeing how necessary this is, especially before elections," he added.
Germany's governing Social Democratic Party (SPD) was also the victim of a cyberattack last year, with email accounts being hacked. The government blamed a Russian military intelligence unit for the hack and summoned a high-ranking Russian diplomat to the Foreign Office in Berlin.
German companies from the logistics, defence, aerospace and IT services sectors were also victims of last year's attack. According to the SPD, the source was an unknown security vulnerability linked to Microsoft software.