National Security Council secretary-general Thawip Netniyom has sent a third request to Laos for the arrest of hardcore red-shirt leader Wuthipong "Kotee" Kochathamakun, who is said to still be active there.
The request was sent again because Lao authorities had not been responsive to previous requests, Gen Thawip said on Friday.
The NSC secretary-general made the first request in person when he visited Laos and then sent a formal letter to Lao authorities himself. Laos remained silent.
Gen Thawip sent another letter of request early last month, which also drew no response, and has now sent a third written request.
Gen Thawip described Mr Wuthipong as being a dangerous person because he had violated the lese majeste law and allegedly planned attacks on national leaders.
"We have found that his movements continue and we believe he remains in Laos and did not flee to another country," the NSC secretary-general said.
The government accused Mr Wuthipong of being opposed to the monarchy and of agitating for a change in the form of government in the country. He is wanted on a total of 21 arrest warrants for a series of serious offences.
He was accused of buying a large cache of weapons with donated money and using them during political demonstrations by the United Front for Democracy against Dictatorship (UDD) and clashes with other demonstrators in 2010 and 2014.
He has also been linked to a major weapons seizure on March 18, which resulted in nine suspects being charged with terrorism among other offences. One of the locations raided was a Pathum Thani house that Kotee used as a base when he was still running a red-shirt radio station in Thailand.
Some opponents of Kotee have speculated on a connection between the red-shirt hardman and fugitive former prime minister Thaksin Shinawatra. That led to Thaksin, through his lawyer, filing defamation and computer crimes complaints against a TV programme anchor and commentator.