The judges, Parn Jantaraparn, Jumpol na Songkhla and Sakdi Techacharn, sued in 2003 after the impeachment proceedings against them were aborted.

The three claimed damages from Sanan and 59 other defendants, including former Senate speaker Manoonkrit Roopkachorn, for alleged failure to verify about 60,000 signatures solicited to endorse the impeachment proceedings.

In its ruling, the court said the defence had presented convincing evidence about reasonable efforts to check the names before handing them to the authorities.

"The defendants collected and checked signatures by practical means available using common sense," the verdict said.

After receiving the impeachment petition from Sanan, Manoonkrit made further checks by appointing a 10-member panel to verify the names, it added.

The petition was later dropped because it fell short of the required 50,000 signatures after checking with records beyond the defendants' control, it said.

Prosecution lawyer Nakrit Sawetanan said his clients would appeal on grounds that Sanan and Manoonkrit had not spent sufficient time on checking the names.