The Election Commission (EC) has apologised after the online advance voter registration system crashed on Sunday, the last registration day.
However, the EC stopped short of saying if it plans to extend the registration period because of the glitch, which left many unable to secure their advance voting rights.
Sawaeng Boonmee, EC secretary-general, said on Monday the problem was first detected around 9pm on Sunday after the system was overwhelmed by the sheer number of people seeking to register for advance voting at the same time.
The system -- comprising the EC's dedicated registration website and the Smart Vote application -- can handle 4,000 registrations per second, according to the EC's technical division.
But on Sunday night, the incoming traffic slowed the system considerably until it ground to a halt, Mr Sawaeng said, adding the EC's registration office was working to fix the problem.
The EC is considering manually contacting individuals who attempted to register for advance voting on Sunday to find out where they plan to vote so arrangements can be made to suit their needs, he said.
Many social media users are calling for more time to register for advance voting.
Mr Sawaeng, however, declined to say if the registration period will be extended.
He said the EC would see what it could do to fix the system and accommodate the affected voters.
The EC secretary-general said more than two million voters have successfully registered for advance voting, comparable to the 2019 election.
For overseas voters, Mr Sawaeng said the EC is working closely with the Foreign Affairs Ministry, Airports of Thailand, and the Department of Consular Affairs to ensure a smooth advance voting process.
All relevant agencies have also reviewed the procedures for overseas voting to prevent a repeat of an incident in 2019, in which a diplomatic bag containing ballots cast in New Zealand arrived late and ended up not being counted.
The Foreign Affairs Ministry is working to ensure every marked ballot will arrive back in the country in time to be counted, he said.
Between March 25 and April 9, 2,153,450 people have successfully registered for advance voting -- 111,069 of whom are planning to cast their ballots overseas.