Federal prosecutors in Mexico City announced yesterday they would seek to overturn the city’s new gay marriage law on the grounds that it is unconstitutional.

A statement from the federal Attorney General’s Office said the law “violates the principle of legality, because it strays from the constitutional principle of protecting the family”.

The law is due to come into effect in March, after being approved in December. It also includes specific rights for gays to adopt and is the first such law in Latin America.

Mexico City previously allowed civil unions but did not allow gay couples to adopt.

Mayor Marcelo Ebrard of the progressive Democratic Revolution Party refused to veto it, despite calls for him to do so from the Mexican Catholic Church and the conservative National Action Party, which said it would appeal to Mexico’s Supreme Court.

The Roman Catholic archbishop of Mexico, Cardinal Noberto Rivera Carrera, deemed the bill “immoral” and “reprehensible” last month.

City officials have said action from the Attorney General’s Office would not prevent the law coming into force.

According to On Top Magazine, city legal advisor Leticia Bonifaz told the Excelsior newspaper that she was “totally confident that this is an issue of fundamental rights”.

In December, two gay men from Argentina had Latin America’s first gay marriage.

Jose Maria Di Bello and Alex Freyre married in Ushuaia, the capital of Argentina’s Tierra del Fuego state where sympathetic state officials backed their bid.