UPDATE 3-Santander bank plans to list part of Mexico unit

* Bank will list up to 25 pct of Mexico unit

* IPO could raise up to $4 billion - IFR

* Bank says no decision yet on size of listing

* Shares to trade in Mexico, New York

MADRID/MEXICO CITY, Aug 16 (Reuters) - Spain's Banco Santander said on Thursday it will register plans in coming days to list up to 25 percent of its Mexican unit in what could be the country's largest ever initial public offering.

In documents filed with the Mexican stock exchange, the bank gave no price range for the offer but sources told Thomson Reuters' IFR that Santander Mexico's IPO could range between $3 billion and $4 billion.

The euro zone's biggest bank, which is battling recession at home and a debt crisis that is spreading across the common currency bloc, has multiple operations in Latin America.

Santander will sell close to 25 percent of its Mexican unit, with just over 6 percent being sold in Mexico and 18.7 percent abroad, according to a filing with Mexico's stock exchange.

The bank said in a statement issued in Madrid it would retain a majority stake.

It will list the shares on the Mexico City and New York stock exchanges, and will market the offering jointly with its Santusa Holding subsidiary.

Spanish builder Obrascon Huarte Lain floated its Mexican unit in November 2010 in a $910 million listing that was the biggest in Mexico in nearly two decades.

Santander has operations in Argentina, Brazil, Mexico, Chile, Peru, Puerto Rico and Uruguay, a customer base of 41 million and over 6,000 branches. Latin America makes up nearly a fifth of the group's assets and 52 percent of its net profit.

It listed part of its Brazilian unit in 2009.

The company shelved plans to list its UK business in June this year but still expects to spin it off and float it in London either next year or in 2014.

Santander shares closed up 0.2 percent at 5.61 euros, compared with a 4.05 percent rise in Madrid's blue-chip IBEX index.