Germany's top court ruled Friday that the military may conduct combat operations within the country's borders in case of a terror attack of "catastrophic proportions", ending a post-war taboo.

The Federal Constitutional Court said the Bundeswehr armed forces could deploy under strict conditions in case of an assault with the potential for scores of casualties, in a decision largely welcomed by the government.

The deployment of troops in Germany is only acceptable in "states of emergency of catastrophic proportions," 15 of the 16 judges ruled, but never "in reaction to the threat posed by demonstrating crowds".

The use of combat weapons is only acceptable "as a last resort" and must be approved by the federal government, not simply delegated to the defence minister.

Shooting down a hijacked passenger plane with civilians on board remains illegal after the ruling but fighter jets could attempt to force an aircraft to land with warning shots.

The decision brings Germany broadly in line with European allies including Italy, France and Britain.

The ruling marked a reversal of a decision by the same court in 2006 and was a response to complaints from two of Germany's 16 federal states.

The German government had attempted to allow more flexibility in its response to a possible terror attack in the wake of the September 11, 2001 suicide hijackings in the United States, and lend backing to police forces.

But it ran into resistance from the court, which cited strict restrictions on the deployment of the military in Germany set down in the constitution in light of the abuses by troops and paramilitary forces in the Nazi period.

Fears also ran deep that the armed forces could be used as an instrument of political power.

The decision effectively broadens Article 35 of Germany's Basic Law which only permits a domestic military deployment in case of a natural disaster or a particularly serious accident.

Defence Minister Thomas de Maiziere and Interior Minister Hans-Peter Friedrich welcomed the decision in a joint statement.

"Ensuring the security of our citizens, particularly in extreme cases, is one of the most important duties of our state," they said.

The ruling "fills a legal void", added defence affairs expert Ernst-Reinhard Beck of the ruling conservative Christian Union parties' parliamentary group.

However Justice Minister Sabine Leutheusser-Schnarrenberg stressed the importance of the "separation between internal and external security" and noted that "not everything that is constitutionally possible is politically right".

The opposition Social Democrats welcomed the "last resort" restrictions imposed by the court, home affairs expert Michael Hartmann said.

But the far-left party Die Linke warned that the decision "opens the door to a further militarisation of home affairs and hollows out democratic rights".

In a dissenting opinion, Judge Reinhard Gaier said the court had gone too far and that the government should seek an amendment to the constitution if it aimed to deploy the military on German soil.