The first case of Beta variant, originally detected in South Africa, has been found in Bangkok, a source said.
An initial investigation found that the infected patient is linked to an infection case in Narathiwat, according to the source.
Department of Medical Sciences director-general Supakit Sirilak will hold a press conference on the issue on Monday.
The Beta variant found spreading in the deep South of Thailand is believed to have come from Malaysia.
Eleven new Covid-19 clusters were found in eight provinces on Sunday, with authorities keeping a close watch on factories, markets and construction camps in high-risk areas.
The 11 new clusters -- four of which were reported in Bangkok -- brought the country's total number of active Covid-19 clusters to 111. These include a construction camp in Huai Kwang, a bakery factory in Lat Krabang, a construction camp and a hotel in Bang Phlat district.
The seven remaining clusters were reported at a garment factory in Samut Sakhon's Krathum Baen district; a market in Nonthaburi's Pak Kret district; a market in Chon Buri's Muang disrict; an electronic devices factory in Ayutthaya's Bang Pa-in district; a mechanical parts factory in Lop Buri's Muang district; a natural resource office in Suphan Buri's Muang district; and a police training centre in Surat Thani's Muang district.
Infections in Bangkok and its surrounding provinces are showing no sign of letting up and the situation in some southern provinces is worrisome, a Centre for Covid-19 Situation Administration (CCSA) meeting was told.
The meeting, chaired by National Security Council chief Gen Natthapol Nakpanich, found that cases in the northern and southern regions were mostly linked to social gatherings, while most of the infections in the eastern and central regions were reported in factories and large-sized markets.
The country on Sunday reported 42 more Covid-19 fatalities and 3,995 new cases, including 45 in prisons and 35 imported cases, which brought the accumulated toll to 1,912 deaths and the number of cases since the start of the pandemic to 244,447.
According to the Department of Corrections, the number of inmates infected with the virus stood at 35,980 and 4,377 were being treated.
Of the imported cases, 31 were from Cambodia and while travellers from Uganda, Kenya, Malaysia and Myanmar accounted for the rest.
Bangkok logged the highest number of both deaths and new transmissions on Sunday, registering 137 new patients and 27 fatalities. Meanwhile, the surrounding provinces of Samut Sakhon saw 289 cases, Samut Prakan 231 cases and Nonthaburi 195 cases.
Chachoengsao, Nakhon Pathom and Pathum Thani logged two fatalities each on Sunday, while one death were reported in Phetchaburi, Chon Buri, Nakhon Ratchasima, Nonthaburi, Prachin Buri, Pattani, Yala, Udon Thani and Uthai Thani.
Meanwhile, the number of "white" provinces with zero cases was down to six, namely Lamphun, Lampang, Phrae, Mae Hong Son, Bung Kan and Mukdahan.
The surge in Covid-19 infections in Greater Bangkok and the southernmost provinces prompted the government to enforce stricter measures in the provinces for one month starting today.
Meanwhile, 241 people were arrested for illegal entry on Sunday, 122 from Cambodia, 56 from Myanmar and 14 from Laos.