The share index fell 0.73 percent to end at 6073.17, its lowest close since June 26.
Moody's cut its credit rating outlook for Sri Lanka from positive to stable on Tuesday, adding to the bearish sentiment.
Turnover was at 738.1 million rupees ($5.65 million) on Thursday, less than this year's daily average turnover of about 1 billion rupees, according to provisional exchange data. John Keells lost 2.2 percent to 253.90 rupees. ($1 = 130.6250 Sri Lanka rupees)
(Reporting by Shihar Aneez; Editing by Simon Cameron-Moore)
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