Heavily-indebted Greece is heading toward a snap election next month and global financial markets have swooned on fears the country could reject terms of an international bailout and pull out of the euro.

"There is no legal exit clause for a country that wants to leave the euro zone, so if it occurs, it is going to be necessarily traumatic, and have global repercussions," Ortiz told Reuters at a banking conference in the Mexican resort of Acapulco.

"Of course this will affect us as the consequences of Lehman's bankruptcy affected the world, only this will probably have an even bigger impact," he said.

The collapse of U.S. investment bank Lehman Brothers in late 2008 sparked Mexico's deepest recession since the 1995 Tequila Crisis.

Ortiz said Mexico's was most vulnerable to a potential freeze in financing for international trade shipments.

"In the case of Mexico, fortunately we do not depend on cross border flows for the operation of the banking system," said Ortiz, who led Mexico's central bank from 1998 to 2009 and is now chairman of the board of Mexican bank Banorte.

"The cross border flows on which we depend have to do with the financing of international trade. If this halts, obviously there are real repercussions."

Euro area banks as a whole provide 36 percent of global trade finance loans, and French and Spanish banks together account for more than two-fifths of trade finance loans in Asia.

Mexico's banking system is dominated by units of major global banks, including Spain's BBVA, but local subsidiaries are well capitalized by local deposits and are not dependent on financing from their parent companies.

Ortiz, who is also a former chairman of the Bank for International Settlements (BIS), said a collapse in trade financing could end up dragging the United States into recession.

"A double dip (recession) would obviously hit the whole world," he said.

Mexico sends nearly 80 percent of its exports to its northern neighbor.

BIS figures show new trade finance loans fell 4.6 percent in the fourth quarter of 2011 from the previous quarter.

Trade finance, which is highly short-term - usually 30 or 90 days - is often one of the first forms of lending to be cut back in difficult times. Trade finance fell about 10 percent between October 2008 and January 2009, according to IMF calculations.

By Michael O'Boyle and Tomas Sarmiento