SEOUL, June 2 (Reuters) - South Korea said on Monday it had decided to delay publishing rules making it easier to import U.S. beef, the final step ahead of a full resumption of American beef imports for the first time in more than four years.

South Korea, once the third-largest importer of U.S. beef until a 2003 outbreak of mad cow disease in the United States, planned to publish relaxed rules on U.S. beef imports on Tuesday after delaying the resumption of quarantine inspections from May 15 due to mounting public safety concerns over U.S. meat.

"We've decided to accept a request by the Grand National Party to delay it," a farm ministry spokesman told Reuters.

He declined to comment how long the delay would last.

The delay comes as thousands of South Koreans have taken to the streets in nightly protests to demand renegotiation with Washington after Seoul agreed in April to fully open its market to U.S. beef imports.

South Korean President Lee Myung-bak, who came to power in February with a record victory margin, has been surprised by the protests and his popularity has plummeted over the decision to import U.S. beef.

In doing so, he removed a major obstacle to U.S. Congress agreeing to a separate free trade deal that studies say could boost annual $78 billion two-way trade by about $20 billion a year.

But critics said the decision, announced during his visit to the United States in April and just before he met President George W. Bush, was a move to please Washington.

Under the deal, Seoul agreed with Washington to accept all cuts of beef from cattle of any age, while other U.S. trading partners such as Japan still won't do so because of concerns over mad cow disease.

Prior to the agreement, South Korea only allowed in boneless beef from cattle under 30 months of age. South Korea had rejected bone-in beef and certain other material, such as spinal columns and brains, which it says pose a higher risk of mad cow infection.

President Lee apologised last month for ignoring public health concerns and promised to restore the ban if there was a fresh outbreak of bovine spongiform encephalopathy or mad cow disease. (Reporting by Miyoung Kim, Editing by Peter Blackburn)

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