Floodwater in some provinces south of Chai Nat, including Bangkok's adjacent Nonthaburi, could rise as much as 30cm as the Chao Phraya barrage faces heavy pressure from increasing run-off from the north.
The barrage will release even more water, despite authorities' promises to keep the discharge in check.
However, authorities insist that Bangkok will still be saved from flooding.
Royal Irrigation Department chief Somkiat Prajamwong held an urgent meeting Monday to assess the water situation as more run-off from the North has reached the barrage.
He said the Chao Phraya River's Ping, Yom and Nan tributaries were swelling due to heavy rainfall, forcing the barrage to release more water.
The RID will increase the water release to reach 2,700 cubic metres per second on Tuesday, from 2,600 cu/m per second Monday. The new level of the water release will be maintained for a week.
The 2,700 cu/m per second is close to the critical level at 2,800 cu/m per second that the barrage can keep the runoff under control.
Still, the amount is far from the level of water release which the barrage made during the massive flooding disaster in 2011 at more that 3,600 cu/m per second.
The RID earlier pledged to keep the discharge rate at 2,600 cu/m per second but the latest run-off had left it with no option but to lift the water release, Mr Somkiat said. He did, however, insist the discharge from the barrage will be gradual, to minimise its effects.
He said the barrage would not increase the discharge any more, although the department has sent out warnings to governors of downstream provinces already ravaged by floods to brace for higher inundation.
Mr Somkiat said floodwater in Chai Nat, Sing Buri and Ang Thong could rise by 25cm on average while Nonthaburi could be hit hard due to high tides. In other Central Plains provinces, such as Suphan Buri and Ayutthaya, the floods were exacerbated by downpours of recent days.
The RID chief said the priority now was to speed up the release of excess water in the Chao Phraya River into the Gulf of Thailand. If more persistent heavy rain fell in downstream provinces, the Chao Phraya River could be "choking with water" as all the 12 catchment fields in the Central Plains region are full after part of the floodwater had been diverted to them.
Rewat Prasong, deputy governor of Ayutthaya, one of the Central provinces heavily ravaged by floods, sent an urgent message to local leaders and people in six inundated districts, warning them to brace for even higher water and making necessary preparations.
The districts are Phak Hai, Sena, Bang Ban, Phra Nakhon Si Ayutthaya, Bang Pa-in and Bang Sai. In many flooded locations in worst-hit Bang Ban and Sena districts, the water has risen to 2.5 metres.
The RID, meanwhile, has set the amount of the barrage's discharge deemed critical for downstream provinces at 2,800 cu/m per second.
At that level, the water could threaten to flood areas in Bangkok on the Chao Phraya River banks as well as some interior communities living close to the networks of waterways connected to the river.
Deputy Bangkok governor Chakkraphan Phewngam has allayed concerns of mass flooding in the city, saying the Bangkok Metropolitan Administration (BMA) has anti-flood measures in place for before, during and after the late King's royal funeral ceremonies.
To protect the old city quarters against water, more than 44,000 sand bags will form an embankment along half a kilometre along the waterways in flood-prone areas with 11 major water pump stations in areas around Sanam Luang and the Grand Palace working at maximum capacity.
In Chai Nat, the RID is redirecting some water from the rising Chao Phraya barrage into the Tha Chin River to ease the hardship of people living in downstream areas that are already flooded.
However, this caused the Tha Chin River to overflow, inundating at least 50 houses along the river in Wat Sing district of this Central Plain province.
Meanwhile, Runoff from Nakhon Ratchasima has poured into the Moon River, which has burst its banks in several districts in neighbouring Buri Ram province. In Khan Dong district more than 2000 rai of farmland has been flooded.
Farmers from the four villages -- Yang Thalae, Muang Thalae, Ngiew and Khaen Thalae -- on Monday used boats as they rushed to harvest their grain in fields covered by more than a metre of water.
In Khon Kaen, flood run-off has burst the Lam Huay Phra creek which leads directly to the inner area of the Muang district, after the earthen dykes built along Nam Phong-Khon Kaen road in Nam Phong district collapsed Monday afternoon.
The incident has prompted workers from the Department of Rural Roads to begin building a flood wall with oversized sandbags across the creek but the effort wasn't successful as the creek was about 10m deep and 50m wide.
Soldiers and local Disaster Prevention and Mitigation officers were sent to help evacuate the residents affected by the sudden floods.
Pisamai Dardsel, 40, a resident of Ban Nong Hin in tambon Sila in Muang district whose house is located in a paddy field, said she called local authorities for help after her house was flooded with 1m deep water.
Earlier in the day, residents in Ban Bueng E-thao village in the same tambon that has been flooded for eight days gathered to protest against the management of the flood waters which resulted in their houses being inundated.
The protesters demanded local administrative authorities stop blocking the run-off from draining into Huay Pla Lai creek, a measure aimed at preventing the flood waters from flowing into the direction of the downtown Muang district.
The residents insisted that blocking the water was worsening floods in their areas.
Provincial governor Somsak Jantrakul later visited the protesting villagers and promised to release water in the creek, which satisfied protesters who then agreed to disperse.