By Ralph Jennings

TAIPEI, Oct 28 (Reuters) - Taiwan's decision to lift a ban on U.S. beef imports has handed President Ma Ying-jeou his biggest crisis since the government's perceived slow response to a deadly typhoon in August.

Taiwan announced on Friday it would reopen markets to U.S. bone-in beef, most likely in November, ending a six-year import ban that was in place over fears of mad cow disease.

The flap is not expected to topple Ma from office but could see a head or two roll in the cabinet, with more fallout if mad cow disease resurfaces, analysts say.

Importers, the main opposition party, the public and ruling party KMT mayors from two major cities have questioned the decision despite repeated government assurances the beef is safe.

Popular approval of Ma, elected by a landslide in 2008, has sunk to 33 percent over the beef decision, according to a newspaper poll, as local ruling Nationalist Party (KMT) leaders publicly question his government's decision.

Beef is shaping up as Ma's second biggest challenge after accusations that his administration responsed too slowly to an August typhoon that killed hundreds of mountain villagers.

"The beef issue so far doesn't appear well handled," said Lin Chong-pin, strategic studies professor at Tamkang University in Taipei. "A policy enunciation wasn't made. There was no expectation of this public reaction."

Taipei Mayor Hau Lung-bin, a Ma protege, went on television on Tuesday waving a no-beef placard.

"We must reiterate that the only way to solve the problem completely is to take a multi-partisan legislative stand ... and strongly demand that the government renegotiate the U.S. beef issue," the opposition Democratic Progressive Party (DPP) said in a statement on Wednesday.

Taiwan banned U.S. beef in December 2003 and allowed boneless beef from 2006. According to the new rules, imports of any meat from cattle under 30 months of age will soon be allowed.

The most recent U.S. mad cow case occurred in March 2006. The caseload worldwide dropped from 37,000 in 1992 to under 300 in 2006, according to the World Organisation for Animal Health.