Activists and workers' unions have demanded land development plans be immediately excluded from the terms of reference of the high-speed railway set to link Don Mueang, Suvarnabhumi and U-Tapao airports, alleging it would monopolise involvement in the megaproject down to "a few large firms."

Srisuwan Janya, secretary-general of the Association to Protect the Thai Constitution, said on Sunday that it is "unnecessary" for land development rights to be given to the firm which wins the operation rights for the Eastern Economic Corridor (EEC) railway project.

The tender, released in June, will grant the winning firm both operation and land development rights under a 50-year concession. The tender papers, which cost one million baht, were purchased by 31 firms before the deadline early last month.

The State Railway of Thailand (SRT) is set to announce the winning bidder of the 200-billion-baht megaproject before the end of the year.

"The current terms can pave the way for corruption and malfeasance, since it provides an unfair and non-level playing field for a few select firms who have the power to bid for such a contract," Mr Srisuwan claimed. "These firms will just be receiving the land around the railway as an added bonus at cheaper rates, compared to the actual, substantially higher value of such land."

As part of the railway project, the SRT must hand over the land it owns around the Bangkok-Rayong route to the winning firm, including 150 rai in the Makkasan area. Mr Srisuwan claims the land in the area for the proposed railway deal is being undervalued, citing that the SRT had reported the land in Makkasan was valued at 600,000 baht per square wah. Nearby land, such as the expanse under Nai Lert Park, is valued at 1.8 million baht per square wah. This has distorted the actual value of the 200-billion-baht deal, he added.

Mr Srisuwan said he is planning to go to Government House today to file a formal complaint to Prime Minister Prayut Chan-o-cha, adding that the matter could reach the Criminal Court for Corruption and Misconduct Cases if the complaint gains no headway. The political activist, along with representatives from SRT and other workers' unions, had submitted a complaint regarding the megaproject last month.

"We have to keep doing this until thorough inspections are conducted on these terms, because such inspections cannot be conducted under the present government," he said. "We do not understand why the project cannot wait until after the elections, so that they may be reviewed justly and openly."

The EEC Committee, headed by Gen Prayut, approved the airport-linking railway in late February. The 220km-long railway is scheduled to open in 2024 and will connect Don Mueang to U-Tapao airport in Rayong via the current Phaya Thai-Suvarnabhumi Airport Rail Link.