By Laura Sheeter
BBC News, Riga
MPs in the Baltic state of Latvia have passed a constitutional amendment ensuring that gay couples cannot marry.
Same-sex marriage is illegal under Latvian civil law
The change was supported by a majority of parliament - despite criticism of it by the country's president, prime minister and foreign minister.
Members of the European Parliament said the move was homophobic and backward.
Same sex marriage is already illegal in Latvian civil law, but the constitution will now explicitly state that marriage can only exist between a man and woman.
The amendment will change one paragraph of the constitution relating to the state's responsibility to protect the family.
Latvia's First Party - which proposed the change - says it did so in order to protect the traditional family group from what it calls the "threat" of homosexual lifestyles.
Members of Parliament said they were concerned that since Latvia has joined the European Union, EU laws will enable gay people to gain rights including the right to marry.
There has been a very public debate about homosexual rights since the first Gay Pride march took place in the Latvian capital, Riga, this summer.
The march, which had been criticised by the Prime Minister, Aigars Kalvitis, was marred by demonstrations and anti-gay protestors outnumbered the marchers.
The constitutional change has been condemned by international gay organisations.
The International Gay and Lesbian Alliance said in a statement that it was appalled and seriously concerned by what it called such homophobic developments in Latvia.
It called on the European Union to look into the "disregard of EU principles and laws".
To become law, the amendment must be signed by the Latvian President, Vaire Vike Freiburga, who has not previously supported it, but with so many MPs in favour of the change it will be difficult for the president to reject.