BY ELOY O. AGUILAR
Associated Press

PANAMA CITY, Panama - Panama's president is appealing to his citizens to support the biggest expansion of the Panama Canal since it opened in 1914, making way for huge new container ships that can carry twice as much cargo.

The $6 billion project, which will be put to voters in a referendum later this year, would add a third series of locks big enough to accommodate the world's biggest cargo ships.

The biggest ships that can pass through the canal's current locks are known as ''Panamax'' vessels and can carry 4,000 cargo containers. They barely fit in the locks, which are about 109 feet wide.

The new locks would be just under 180 feet wide, to allow passage of ''Post-Panamax'' ships, which carry 8,000 containers. The government argues the wider locks are key to protecting the role of the Panama Canal and its crucial income-generating capacity -- an argument President Martin Torrijos is expected to make Monday evening in a nationally televised appeal for voters to support the expansion.

The United States returned administration of the canal to Panama in 1999, under a treaty negotiated two decades earlier by Torrijos' father, President Omar Torrijos.

About 13,000 ships passed through the waterway in 2005, paying about $1.2 billion for canal fees and maintenance and other related services.

Still, the project's estimated $6 billion cost would be a big investment for a country whose government budget is $6.5 billion a year. Officials are counting on private bank financing.