Ms Yingluck said Thailand proposed the central province and ancient capital of Ayutthaya as the site for the World Expo 2020 and it is committed to the plan.
She said it was totally groundless that Thailand would withdraw from the candidacy.
The premier's comments came after opposition Democrat Party leader Abhisit Vejjajiva called on the government to show its strong intention to host the international fair as it would be a good opportunity for the country.
A Bureau of International Expositions (BIE) Enquiry Mission met Ms Yingluck before going to Ayutthaya from Jan 28 to Feb 1 to inspect Ayutthaya's potential for hosting the world's largest fair.
However, the government was reportedly reassessing the utility of investing in a bid to stage the World Expo.
Meanwhile, Ayutthaya Governor Witthaya Piewpong confirmed that the province was ready to host the Expo. He said it was not true that the province would drop out from the competition.
Five cities are competing with their own bids for the BIE -- Izmir in Turkey; Yekaterinburg, Russia; Sao Paolo, Brazil; United Arab Emirates’ Dubai and Ayutthaya.
A decision will be made at the BIE General Assembly in November.