BERLIN (Reuters) - Some conservative lawmakers in Germany may take the legalization of gay marriage, approved by parliament last week, to Germany’s top court to ascertain if it is unconstitutional, a German newspaper reported on Monday.

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Germany was taken by surprise by the vote, which was called suddenly, leaving little time for debate. It came after three parties who could enter a coalition with Chancellor Angela Merkel’s conservatives after a September election set the legalization of gay marriage as a condition for their support.

But the same-sex bill faces opposition from some members of Merkel’s conservative bloc and others on the right. Merkel herself voted against it on Friday, but it went through with 393 lawmakers in the Bundestag voting in favor versus 226 against.

Now a group of conservative lawmakers is considering taking the issue to the Constitutional Court, German newspaper Die Welt reported.

The right-wing Alternative for Germany (AfD) party may also file a complaint to the court, said AfD top election candidate Alexander Gauland.

“I’m in favor of taking such a step,” he told Bild am Sonntag newspaper.

Although Germany’s constitution does not explicitly say so, critics argue the term marriage can only apply to a man and a woman.

“For me, marriage in the Constitution is marriage between a man and a woman and that is why I did not vote in favor of this bill today,” Merkel said on Friday.

Merkel is running for a fourth term in a Sept. 24 election. Polls show her conservatives are by far the most popular party but would need a coalition partner - most likely the liberal Free Democrats (FDP), Social Democrats (SPD) or Greens, all of which made legalizing gay marriage a condition for a tie-up.

Johannes Singhammer, a member of the Christian Social Union (CSU) - the Bavarian sister party to Merkel’s conservatives - said he recommended putting the issue to the Constitutional Court to achieve legal clarity.

German Interior Minister Thomas de Maiziere also voted against the move. He told Bild am Sonntag that as a trained lawyer he believed it required an amendment to the constitution, a view shared by a former president of the Constitutional Court, Hans-Juergen Papier, according to other media.

Germany is in the process of unwinding a legacy of virulent homophobia. Earlier this year, parliament agreed to grant compensation to thousands of gay men jailed under a 19th century law that was strengthened by the Nazis and only dropped in 1969 when homosexuality was decriminalized in West Germany.

The same-sex bill is expected to get signed into law after July 7. This makes Germany the 23rd country to legalize same-sex marriage, according to gay rights group GLAAD.