BANGKOK (REUTERS) - A series of Covid-19 clusters among construction workers in Bangkok, including the first local cases of the variant first identified in India, has put a fresh spotlight on migrant labour rights as Thailand grapples with its deadliest outbreak so far.

Migrant workers have been crucial during Bangkok's decade-long construction boom, but they receive limited welfare from the government and their employers, advocacy groups say. Many live onsite in crowded dormitories to save money.

Thailand last week detected its first 36 domestically transmitted cases of the highly infectious B16172 Covid-19 virus variant among people in construction workers' accommodation in Bangkok.

City authorities have prohibited movement of workers at such camps after 11 were among 30 active clusters in the capital.

There are around 409 workers' camps around Bangkok where authorities say 62,169 workers live, about half who are migrant labour.

"Migrant workers are a group that society overlooks," Ms Suthasinee Kaewleklai, Migrant Workers Rights Network (MWRN) coordinator in Thailand, said.

"Many were not confident that they can come in for tests and not be arrested."

The International Organisation for Migration estimates that there are four million to million million migrant workers in Thailand, many working in construction, manufacturing and seafood industries.

Workers from Myanmar were among those worst hit by Thailand's previous coronavirus outbreak in December, the epicentre of which was a province near Bangkok with a high concentration of migrant labourers. The outbreak also saw a sharp rise in anti-Myanmar hate speech on Thai social media.

There are also concerns that illegal border crossings are undermining the country's strict quarantine system for arrivals in the country.

The authorities tightened border crossings at the weekend after three local cases were found of the Covid-19 variant first identified in South Africa, stemming from illegal entry.

Thailand is battling its most severe outbreak yet, fuelled by another highly transmissible variant, B117, first detected in Britain, which authorities believe may have entered Thailand via an illegal border crossing from Cambodia.

Nearly 33,000 illegal migrants had been arrested since July 2020, the defense ministry said Thailand's latest outbreak has accounted for nearly 80 per cent of its 132,213 cases and close to 90 per cent of the 806 deaths recorded so far.