"If you allow a repeat of the same election pattern, then we will always get the same result," General Prayut Chan-O-Cha, dressed in full military uniform, said in an interview aired on two army-run channels late Tuesday.

"I want you to use sound and reasonable judgment to make our country and our monarchy safe and have good people running our nation," he said, apparently endorsing the ruling Democrats, who came to power with army backing.

Parties linked to fugitive ex-premier Thaksin Shinawatra have won the most seats in the past four elections, but the former telecoms tycoon was ousted in a 2006 coup and the courts reversed the results of the last two polls.

Thaksin's youngest sister Yingluck Shinawatra is the main opposition candidate for prime minister and polls show her Puea Thai Party pulling ahead of Prime Minister Abhisit Vejjajiva's Democrats ahead of the July 3 vote.

The Thai army has a long history of meddling in politics, with 18 actual or attempted coups since 1932.

The 2006 military intervention ushered in a period of political instability and bloody unrest, including street clashes between armed troops and protesters in April and May last year that left 90 people dead and about 1,900 wounded.

On the instructions of its commander-in-chief, the military in April filed a complaint against three leaders of the opposition "Red Shirts" for allegedly insulting the royals during rally speeches.

Prayut, a staunch royalist, said the military would stay neutral in the election, but said anybody insulting the monarchy should be prosecuted.

"Based on security intelligence there are widespread violations against the institution," he said.

"Thailand exists today because of the monarchy. The king did not get involved in things outside of his duty. He has worked for more than 60 years.

"He should rest and be relieved to see the stability of the people," he said.