BANGKOK More than 600,000 Thais have been affected by flooding since July and more than a quarter of Thailand's provinces have been inundated, prompting officials to issue landslide warnings and begin evacuation measures on Monday.
Devastating floods in 2011 killed more than 800 people and caused massive disruption to industry, cutting economic growth that year to just 0.1 percent.
Four people have been killed in this year's flooding. More rainfall is expected later this week.
"Due to a heavier-than-usual monsoon season, 21 provinces are now experiencing flooding. We have issued a warning about landslides and have told boats in the Gulf of Thailand to be vigilant," Chatchai Promlert, chief of Thailand's Department of Disaster Prevention and Mitigation, told Reuters.
Parts of Ayutthaya province north of the capital, Bangkok, have been deluged by up to a metre of water, he said. Ayutthaya town is a UNESCO World Heritage Site where several ancient temples were badly damaged in the 2011 floods.
Many huge industrial estates in Ayutthaya and other provinces on a plain north of Bangkok were inundated in 2011.
At least 10 provinces in Thailand's central plains, the main rice-growing region, have been affected by the floods but the extent of any damage is not yet known.
The flooding has hit in harvest season and output is likely to be affected, but there has been no official comment yet.
Plodprasop Suraswadi, a deputy prime minister, said he was confident there would not be a repeat of the 2011 floods, which at one stage threatened to engulf Bangkok.
"Water levels in our dams are low enough to handle any extra rainfall," Plodprasop said.
Municipal authorities have ordered sandbags stacked around shops and homes and extra water pumps have been installed in many areas.
In Prachin Buri province, 135 km (85 miles) east of Bangkok, more than 700 inmates from a prison were evacuated on Sunday and transferred to other facilities nearby after their cells were flooded.
The national disaster department said it was offering assistance to residents in flood-affected areas, including the provision of life-jackets and boats. Those living in low-lying areas have been advised to move to higher ground. (Additional reporting by Apornrath Phoonphongphiphat and Aukkarapon Niyomyat; Editing by Alan Raybould and Paul Tait)