But the court ruled that "the distinction between marriage and registered partnership. can not be upheld at this day and age without discriminating against same-sex couples".
"Because the separation into two legal institutions expresses that people with same-sex sexual orientation are not the same people with different sexual orientation".
The case was heard after a female couple challenged a 2009 law which allowed registered partnerships for same-sex couples, but not marriage.
Lawyer Helmut Graupner, who represented the couple, praised the ruling on social media. " said ILGA-Europe Executive Director Evelyne Paradis, speaking after the verdict was announced".
It said however that the current rules would remain in place until December 31, 2018 unless Austria's parliament changes the law before then.
The Austrian Constitutional Court has ruled that starting in 2019, same-sex couples will have the right to marry.
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Austria is now the 26th country to legalise same-sex marriage.
As it now stands, same-sex couples in Austria had been allowed to enter civil partnerships for the last seven years.
It comes after a case was brought by two women in a registered civil partnership who were denied the right to marry by authorities in the capital Vienna.
The two parties negotiating to form a new government after Austria's October election, the conservative Austrian People's Party and the right-wing Freedom Party, have so far opposed gay marriage. A marriage between women and men needs protection as only these partnerships can create children, he said. "We are very happy", said HOSI chairman Christian Hoegl. "We want to use the opportunity for a renewed call for a fundamental reform of marriage". Same-sex marriage has been legal across the United States since 2015.