BEIJING, Sept 14 (Reuters) - China has warned Internet users to be wary of downloading virus-infected mooncake greeting cards ahead of the traditional Mid-Autumn Festival after a wave of Internet worms hit hard-drives last year.

Mooncakes, sweet pastries with a bean base, are traditionally eaten and given as gifts during the festival, also known as the Moon Festival, a celebration of plenty and togetherness which falls on Sept. 25 this year.

But local Web sites offering electronic versions have become popular in recent years.

"Always scan files over the Internet before you download them," Friday's China Daily quoted an official with southern Guangdong province's Internet supervision bureau as saying.

China's sprawling and unruly Internet experiences regular outbreaks of viruses and worms, often spread through hugely popular online chat tools.

"I was very attracted by the e-card on QQ (a Chinese online chat application), but after I opened it many harmful pages popped up on my computer," the paper quoted PricewaterhouseCoopers employee Pan Yanyan as saying, adding that her hard drive required a complete reformat.

Electronic mooncakes, however, may be safer than the real thing in Guangdong, where only 85 percent of 80 batches of mooncake filling tested met quality standards, according to a report posted on the Web site of the provincial food safety bureau.

Despite finding excessive traces of intestinal bacteria, preservatives and high acidity levels, the authority said consumers could "rest assured" -- 98.1 percent of the finished product on supermarket shelves met standards.

China has been assailed on all sides over export health safety in recent months involving exports ranging from toothpaste, tyres and toys to seafood and drugs.