Days of widespread and heavy rain in the upper part of the central, lower northeastern and eastern regions have also resulted in flash floods in many provinces, affecting farmland and businesses in urban areas.
Flood water from the North has reached Ayutthaya province through the Chao Phraya and Pasak rivers and flowed into the Hantra and Kamang canals.
Sluice gates intended to protect urban areas in the heart of the province, as well as its Uthai district and the Rojana Industrial Park, have not yet been completed.
The canals are overflowing in tambon Hantra and tambon Phai Ling sub-districts.
Water from both canals is also flowing into the Khao Mao canal which leads to the Rojana Industrial Park and the Muang Mai community.
The Royal Irrigation Department closed the sluice gate at the mouth of the Khao Mao canal.
If the water gets through the gate, it would reach the commercial areas of Ayutthaya very quickly.
Industry Minister Prasert Boonchaisuk said he was worried about the situation at Saha Rattana Nakorn Industrial Estate as it was the only one of six industrial estates in Ayutthaya and Pathum Thani where floodwalls have not been completed.
The flood protection system at Saha Rattana Nakorn is only 30% complete.
Floodwalls are in place at the other five facilities, namely the Bang Pa-in Industrial Estate, the Ban Wa (Hi-Tech) Industrial Estate and the Rojana Industrial Park in Ayutthaya, and the Navanakorn Industrial Estate and the Bang Kadi Industrial Estate in Pathum Thani.
"The IEAT [Industrial Estate Authority of Thailand] is monitoring the situation at the three estates in high-risk locations, namely Saha Rattana Nakorn, Bang Pa-in and Hi-Tech," Mr Prasert said.
"I have ordered the IEAT to install big sandbags that can be laid over a distance of 20 kilometres at the Saha Rattana Nakorn Industrial Estate."
Despite flooding elsewhere in Prachin Buri, the newly built Hi-Tech Kabin Industrial Estate in the eastern province remained intact because its compound is elevated.
Suthep Noipairoj, deputy director-general of the Royal Irrigation Department, said water from Nakhon Sawan province has now reached Bangkok and the level of the Chao Phraya River in the capital and nearby provinces is rising quickly.
Koh Kret island in Nonthaburi province, north of Bangkok, is now partially flooded as a result. Koh Kret is located on the river.
Mr Suthep said areas protected with embankments along the river, including those in Bangkok, would not be flooded.
Bangkok governor MR Sukhumbhand Paribatra said 40% of the reservoirs of dams were filled, compared with 90% in 2011 when the country was hit by massive flooding.
The tide in the Chao Phraya River is only about one metre above the mean sea level, so he is confident Bangkok will be safe from flooding.
The Department of Disaster Prevention and Mitigation said flooding has hit a total of 21 provinces, comprising seven in the Northeast, four in the North, seven in the Central Plains, two in the East and one in the South.
Severe flooding was reported Monday in Si Sa Ket, Surin, Prachin Buri, Sa Kaeo and Buri Ram provinces.
Northeast-bound trains to Ubon Ratchathani can go no further than Sikhoraphum station in Surin, as the tracks between Surin and Si Sa Ket are 1.5m under water over a distance of about 3km.
The Department of Highways said highways are flooded in 13 provinces: Chachoengsao, Pathum Thani, Lop Buri, Saraburi, Suphan Buri, Chaiyaphum, Nakhon Ratchasima, Buri Ram, Prachin Buri, Sa Kaeo, Ubon Ratchathani, Phetchabun and Surin.
Eighty percent of highways in the northeastern province of Surin are inundated.
The Meteorological Department predicts more rain in the lower North, the upper part of the Central Plains and the Northeast from tomorrow until Saturday.
Meanwhile, the Royal Irrigation Department said it will increase the discharge rate of the Chao Phraya dam in Chai Nat province to relieve the water run-off from the North. The rate will rise from 2,000 to 2,300 cubic metres per second.