Thailand's junta chief hit out Monday at critics of a controversial court verdict which sentenced two Myanmar migrant workers to death for murdering a pair of British tourists last year.

Zaw Lin and Win Zaw Tun were found guilty on Thursday of killing David Miller, 24, and the rape and murder of Hannah Witheridge, 23, whose battered bodies were found on a beach in the southern diving resort of Koh Tao in September 2014.

The killings have sullied Thailand's reputation as a tourist haven and raised questions over its justice system after the defence accused police of bungling their investigation and using the men as scapegoats -- a charge authorities deny.

The verdicts have sparked anger in neighbouring Myanmar, with daily protests there. The country's powerful army chief has urged Thailand to review the case.

But Prime Minister Prayut Chan-O-Cha angrily dismissed those calls in his first comments on the case since Thursday's verdict.

‘They have the right to appeal, right? Laws all over the world have this. Or should Thai law not have this? Is it the case that we should release all people when pressured?’ a visibly angry Prayut told reporters.

Prayut, who seized power in a coup last year, is known for his mercurial outbursts and often colourful statements.

In the days and weeks after the Koh Tao murders, Prayut ordered police to make swift arrests, fearful of the impact the killings might have on the vital tourism industry.

He frequently aired his own opinions about who might be guilty, saying it was unlikely a Thai could carry out such a grisly murder.

Prayut also later apologised for comments suggesting beautiful foreign women wearing bikinis should not expect to be safe.

Thai prosecutors and police insist their evidence against the men, both aged 22, was rock-solid, including DNA found on Witheridge's body.

At a press conference Sunday police insisted their investigation was above board and hit out at the recent protests in Myanmar.

But the defence, which has vowed to appeal the verdict, disputed the forensic evidence, saying it was improperly collected and processed.

They also accused the police of torturing their clients into signing confessions which they later retracted.

The court in Koh Samui dismissed the defence's torture allegations and ruled that the DNA evidence proved guilt.

Lawyers for the two men say they will appeal.