However, the agency has shied away from immediate full surveillance of the entire party, including phone and email taps, the use of undercover informants and the collection of personal data on MPs.

A report on the move by Berlin's Tagesspiegel daily was confirmed to AFP by sources familiar with the decision ahead of separate Berlin press conferences by domestic intelligence (BfV) chief Thomas Haldenwang and AfD leaders.

The five-year-old Alternative for Germany, the country's biggest opposition party, opposes multiculturalism, Islam and the immigration policies of Chancellor Angela Merkel, whom it labels a “traitor”.

The BfV can place under surveillance individuals and groups, including politicians and parties it considers “extremist” and threatening to the state's liberal democratic order.

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It has in the past placed under surveillance some lawmakers of the far-left opposition Die Linke party, which emerged in part from the former East Germany's communist party.

The BfV has in recent months reviewed inflammatory statements and social media posts of AfD members and was to announce Tuesday that it had officially designated the party a “review case”.

It was also to start full surveillance of the party's youth organization Junge Alternative (JA), which is suspected of having ties with the extremist Identitarian Movement.

And it was to place under surveillance the AfD's most far-right grouping “The Wing” (Der Flügel), led by nationalist Björn Höcke, reported the Tagesspiegel.

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Höcke has sparked outrage with statements on Germany's Nazi past, calling Berlin's Holocaust monument a “memorial of shame” and urging a “180-degree shift” in the country's culture of remembrance.

News of the stepped-up BfV watch comes as a blow to the AfD, which in 2017 elections won 13 percent of the national vote and is represented in all 16 state parliaments.

The populist party hopes to make further gains in European parliament elections in May and in three autumn state polls in Germany's formerly communist east, its electoral heartland.