Calls have grown for tighter security on the US border with Mexico

Two-thirds of Mexicans who migrate, or try to migrate, to the US every year do so without documents, it said.

The report, timed to coincide with the UN International Migrants Day, said almost 30 million people in the US were direct descendants of Mexican migrants.

Illegal immigration is set to be a key issue in the 2008 US presidential poll.

Tricky issue

The illegal entry to the US, via its southern border, of an estimated 500,000 people each year, has led the US government to start building a security fence along the most sensitive areas.

US President George W Bush's attempts to overhaul immigration laws collapsed amid heated debate in Congress earlier this year.

The question of how to address illegal immigration, without alienating the large Hispanic electorate, is a tricky political issue for candidates.