The FDP opposes the naturalization reform initiated by Interior Minister Nancy Faeser.
"Now is not the time to simplify citizenship law," said FDP General Secretary Bijan Djir-Sarai to the "Rheinische Post" on Monday.
"So far there has been no progress on repatriation and combating illegal migration."
Referring to SPD politician Faeser's plans to accelerate naturalization, Djir-Sarai said that the granting of citizenship was the result of successful integration into German society.
"It must not be at the beginning of the integration process."
Union: "First integration, then citizenship"
Like the FDP, the Union was also skeptical about the interior minister's plans.
The German passport should not be devalued, said CDU General Secretary Mario Czaja of the "Rheinische Post".
It must continue to apply: "First integration, then citizenship".
The SPD, Greens and FDP had agreed in their coalition agreement that foreigners in Germany could more easily obtain German citizenship.
Federal Interior Minister Faeser is now driving the issue forward.
A draft law by the Ministry of the Interior provides that people in particular who have already lived in this country for several years can become German citizens more easily.
Instead of the previous eight years, you should be able to obtain citizenship after five years of residence in Germany.
In the case of "special integration achievements", this should even be possible after three years - for example if immigrants have shown special academic or professional achievements or voluntary commitment or have particularly good language skills
In a guest article for the "Tagesspiegel", Faeser wrote: "With the new nationality law, we are therefore creating incentives for integration instead of building up hurdles and demanding long waiting times."