However, the resolution's legality was questioned as all four EC members were present when it was passed and one was reported to have approved the decision by phone.

Earlier a former Democrat Party MP said the party would file a lawsuit against the EC if it went ahead with the new round of registrations for lower house by-elections.

Sirichoke Sopa said the EC would be violating the law if it allowed a fresh round of registrations for the by-elections in seven Songkhla constituencies, despite the fact the lists were closed on April 9.

He dismissed the EC's claim that a crowd of Songkhla residents had stopped minor party candidates from entering the registration centre on April 8 and 9, which would have resulted in Thai Rak Thai's candidates running for seats on their own.

Sirichoke said some of the candidates of the minor parties - Pattana Chart Thai, Better Life and Khon Kho Plod Nee - failed to register because they did not meet the EC's qualification requirements, while the rest did not show the full documentation.

Shortly after the April 2 poll, the EC called for by-elections in 38 constituencies where lone candidates had failed to gain the minimum 20 per cent of votes required.

The Democrats alleged the EC's decision to allow additional candidates to contest the repeat vote was a ploy to help Thai Rak Thai contenders to overcome the 20-per-cent rule.

Thai Rak Thai spokesman Jatuporn Phromphan said the party wanted the EC to reopen registration because he had evidence a radio presenter Chalee Noppawong na Ayutthaya, seen pro-Democrat, had organised a blockade of the registration centre.

Meanwhile, Chalee dismissed Jatuporn's allegation that the crowd blocked the way. EC officials confirmed, he said, the crowd made no attempt to scare off candidates.

But if the EC in Bangkok insisted on another round of registration, his group would hold another protest and file a petition with the Administrative Court to cancel the process, Chalee said.