Deputy Prime Minister Prawit Wongsuwon reported sick and was unable to attend Wednesday's cabinet meeting, which had been postponed from Tuesday.
The meeting was chaired by Prime Minister Prayut Chan-o-cha for the first time since he was suspended from duty for over a month. The suspension was lifted after Constitutional Court ruled that his eight-year tenure as prime minister ends in 2025.
In his absence, the cabinet meetings were chaired by Gen Prawit, as acting prime minister.
Gen Prayut was informed on arriving at Government House on Wednesday morning that Gen Prawit was taking sick leave.
Gen Kongcheep Tantravanich, Gen Prawit's spokesman, said the deputy prime minister had developed a fever after his flood inspection trip to Ayutthaya, Chai Nat, Ang Thong and Sing Buri provinces on Monday.
A doctor examined him on Wednesday morning and said his symptoms were mild. Gen Prawit would be able to visit Pathum Thani province as scheduled on Wednesday, he added.
Government spokesman Anucha Burapachaisri said Gen Prayut and his entourage were scheduled to visit Lom Sak district in Phetchabun province on Thursday.
He would arrive at Phetchabun airport about noon and be briefed about the flooding situation by provincial governor Kris Kongmuang.
He would then go to Moo 11 village in tambon Tan Dieo, Lom Sak district, to meet people affected by the overflow from the Pasak River.
Gen Prayut would later meet flood-hit people in Lom Sak town at Lom Sak museum and Dong Tan public park before returning to Bangkok.
Mr Anucha said six districts of Phetchabun had been hit by flooding. They were Muang, Wichian Buri, Si Thep, Lom Sak, Bung Sam Phan and Nong Phai districts.