German Chancellor Angela Merkel's centre-right CDU on Monday said the party will choose its new leader at a congress next month that will be held online because of the coronavirus pandemic.

The January 15-16 meeting will be the CDU's first digital congress and the format was chosen "in close coordination" with the three men vying for the top job, the CDU said in a series of tweets.

Corporate lawyer Friedrich Merz, centrist foreign affairs expert Norbert Roettgen and Armin Laschet, the premier of Germany's most populous North Rhine-Westphalia state, are all vying for the post.

The winner will be picked by 1,001 CDU delegates on the second congress day. The choice will be confirmed by postal ballots afterwards that will not feature the names of the two unsuccessful candidates, the CDU said.

The chief of the Christian Democratic Union traditionally leads it and its smaller Bavarian sister party the Christian Social Union (CSU) to the polls.

The chosen candidate would be the frontrunner to replace Merkel as chancellor should the conservative bloc win next year's general election on September 26.

Merkel, who has been in power since 2005, plans to step down after finishing her fourth term at the helm of Europe's top economic power.

Her protegee Annegret Kramp-Karrenbauer took over as CDU leader in 2018.

But she resigned just a few months into the post over her handling of a regional election scandal, throwing the race for the top job wide open.

The battle has since been overshadowed by the pandemic and the congress has been postponed twice already.

The decision for a virtual congress comes as Germany struggles to cope with a second coronavirus wave that has brought record daily infection numbers and deaths.

The country is set to go back into a partial lockdown from Wednesday, with schools and non-essentials shops to stay closed until at least January 10.

The crisis has made it hard for the three CDU hopefuls to create real momentum for their leadership bids, while surveys suggest many Germans would prefer to see Bavarian premier Markus Soeder from the CSU as their next chancellor.

Soeder's robust response to the pandemic has won him widespread praise and given him a national spotlight. But he has repeatedly ruled out running, insisting that his place "is in Bavaria".

Merz, a 65-year-old millionaire and old Merkel rival, is the favourite choice among CDU voters, surveys show.