South Korean companies' business environment outlook soured for next month due to fewer work days and concerns about protectionism by major trading partners, a local think tank said Friday.

The business survey index for May fell to 91.7, down from 102.3 tallied a year ago and 93.3 a month earlier, according to the Korea Economic Research Institute, the think tank unit of the Federation of Korean Industries. 

"The outcome was mainly due to fewer working days, political uncertainties in regards to the presidential election on May 9, and growing worries about protectionism in the US and China," KERI said in a statement. 

A reading below 100 means pessimists outnumber optimists, while a reading above the benchmark means the opposite.

The index remained below the benchmark since May 2016 when it reached 102.3, although the reading for May 2017 marked the highest in five months.

KERI sent questions to the country's top 600 companies in terms of sales and 436 responded in a survey carried out over 10 days that ended Monday. (Yonhap)