The German president has said a new law allowing for hijacked airliners to be shot down may violate the constitution.
Koehler was backed by fellow opposition party members
Horst Koehler passed the law, but warned it might contravene human rights clauses by allowing passengers to be "sacrificed" to protect others.
He called for a legal review on the grounds that the constitution prohibits the government from killing citizens.
Interior Minister Otto Schily dismissed Mr Koehler's doubts and said he was wrong to publicly question the law.
President Koehler said he had signed the law into effect because it contained other measures vital to air safety - such as tighter air-traffic controls and more powers for pilots to restrain disruptive passengers.
In a statement issued by his office, however, he expressed "serious doubts" over whether the new law "is compatible with the right to life and physical integrity guaranteed by the constitution".
Germany's conservative opposition, of which the president is a member, welcomed the statement and criticised the government for rejecting calls to change the constitution.
A spokesman for the Christian Democrats said the party would mount a legal bid for the constitution to be changed in order to allow for attacks on hijacked aircraft.
The law was drafted after the 11 September 2001 attacks on New York and Washington, in which hijackers used airliners as missiles, killing thousands of people on the ground.
Calls for it to be implemented grew after a German man briefly stole a light aircraft in January 2003 and threatened to crash into Frankfurt, before landing safely at an airport.