The Finance Ministry's think tank attributed the downward revision to the poorer-than-expected export outlook.
It noted that the government will need to speed up budget disbursement in the last three months of this year, to boost the economy.
Recently the National Economic and Social Development Board (NESDB) made a downward revision of the economic growth forecast for 2013 to 3.8-4.3 per cent, the second revision this year, following its May revision to 4.2-5.2 from the original forecast of 4.5-5.5 per cent.
Despite the poor outlook, the NESDB expected that investment and tourism should buoy the economy in the latter half, as well as the government's economic measures. Thailand's tourist arrivals should hit 26.2 million this year, against a previous target of 24.7 million.