The Fiscal Policy Office (FPO) has further trimmed its full-year economic growth forecast to 3% after a worse-than-expected export showing in the first half.
The new growth projection is based on an assumption that export volume in terms of US dollars will fall by 4% this year, director-general Krisada Chinavicharana said.
In April the Finance Ministry's think tank slashed its 2015 growth forecast to 3.7%, down from 3.9% in January and 4.1% last October. The previous 3.7% estimate had assumed export growth of 0.2%.
Exports have long been the mainstay of the Thai economy, accounting for 70% of GDP.
The slow pace of the global recovery and Thailand's structural problems such as a relatively high minimum wage, limited manufacturing technology and removal from Europe's Generalised Scheme of Preferences list have contributed to the export slowdown, Mr Krisada said.
The FPO also cut its economic growth outlook for 15 major Thai trade partners to 3.61% from 3.76%.
The Commerce Ministry on Monday said exports were down for a sixth straight month in June, sliding 7.87% year-on-year to US$18.2 billion. First-half exports fell by 4.84% to $107 billion.
The FPO's latest economic growth projection is on a par with the Bank of Thailand's forecast of 3%, but the central bank predicts an export decline of 1.5%.
The National Economic and Social Development Board (NESDB) is also set to cut its economic growth forecast to 3-4% next month when it reports second-quarter GDP.
The NESDB in May said the economy grew by 3% in the first quarter, an improvement from 2.1% in last year's fourth quarter.
Mr Krisada said tourism should drive economic growth to 3% in 2015 even though dismal exports were expected to continue.
The FPO raised its forecast for the number of foreign tourist arrivals to 29.9 million this year, up by 20.8% year-on-year, from 29.4 million predicted in April.
It expects tourism will contribute an estimated 1.42 trillion baht in revenue, up 24% from 2014.
For the six months through June, the number of foreign tourists visiting Thailand shot up 34.1% on the year-earlier period to 14.7 million. In terms of revenue, 320 billion baht was generated from foreign tourists arriving in Thailand during the first half.
Public spending and investment are other forces that will drive full-year economic growth to 3%, Mr Krisada said.
The FPO estimates 72% of the 449-billion-baht investment budget set for the fiscal year ending Sept 30 will be taken out.
For the October-June period, disbursement of the public investment budget stood at 48.4%.
The baht is also expected to weaken by 4.5% against the US dollar this year but gain 3.2% against the currencies of 15 major trade partners.
Kulaya Tantitemit, the FPO's executive director for macroeconomic policy, said slumping oil prices would be a boon to the Thai economy.
The FPO forecasts oil prices averaging $60 a barrel.
The Bank of Thailand's Monetary Policy Committee is likely to stand pat at 1.5% at next Wednesday's policy rate call, Ms Kulaya said.
For his part, Deputy Prime Minister MR Pridiyathorn Devakula remains positive that Thai shipments will avoid contraction this year.
He sees flat growth at worst, with the overall economy growing by 3% on the strength of tourism.
"Thai exports are expected to pick up in the fourth quarter thanks to higher competitiveness of Thai exporters in light of the weak baht," MR Pridiyathorn said.
He said higher private investment would support the Thai economy, citing a sharp increase in investment application approvals by the Board of Investment (BoI) in the first half.
The BoI recently said it had approved 1,254 projects worth a combined 413 billion baht during that paeriod.
Industry Minister Chakramon Phasukvanich said capacity utilisation in the industrial sector was expected to rise from an average 60% in the remaining months of the year.
Three tyre factories are in the process of being built, with eight to 10 more factories poised to begin construction.
Car makers awarded investment privileges last year for the second phase of the eco-car scheme are also committed to their investment, Mr Chakramon added.