A court of appeals in Thailand has handed a political activist a record sentence of 50 years in prison for insulting the monarchy.

The court found the activist, Mongkhon Thirakot, 30, guilty of 25 violations of the law, a lawyers' group said Thursday.

Mongkhon had originally been sentenced last year to 28 years in prison by the provincial court in the northern province of Chiang Rai for 14 of 27 posts on Facebook for which he was charged.

Mongkhon was found guilty by the Northern Region Court of Appeals in Chiang Rai on Thursday not just in the 14 cases, but also in 11 of the 13 cases for which the lower court had acquitted him, the group Thai Lawyers for Human Rights announced.

The court of appeals sentenced him to an additional 22 years in prison, bringing his total to 50 years.

Technically, he had been given a prison term of 75 years, but the sentence was cut by one-third in acknowledgement of his cooperation in the legal proceedings.

The law on insulting the monarchy, an offence known as lèse-majesté, carries a prison term of three to 15 years for each count.

It's often referred to as Article 112 after its designation in Thailand's Criminal Code.

Critics say the law is a tool to quash political dissent.

Student-led pro-democracy protests beginning in 2020 openly criticised the monarchy, previously a taboo subject, leading to vigorous prosecutions under the law, which had previously been infrequently employed.

Since those protests, more than 260 people have been charged with the offence, according to the lawyers' group.

The court of appeals reversed the lower court's acquittals on the basis the law applied in instances where it wasn't the current monarch or his immediate family who was being referred to, which had been the standard for many years.

However, as lèse-majesté prosecutions became more common over the past decade, a court case set a precedent by finding that past rulers were also covered by the law.

Theerapon Khoomsap, a member of Mongkhon's defence team, confirmed the account given by the Thai Lawyers for Human Rights.

He said that the verdict didn't come as a surprise to him, and his team would appeal the case to the Supreme Court. However, Mongkhon's application to be allowed to continue to be free on bail was denied.

The previous record prison term for the offence belonged to a former civil servant identified by the lawyers' group only by her first name, Anchan.

She was found guilty in 2021 on 29 counts for audio clips on Facebook and YouTube with comments deemed critical of the monarchy.

The court initially announced her sentence as 87 years, but cut it in half because she pleaded guilty.

On Wednesday, prominent human rights lawyer and political activist Arnon Nampa was sentenced to four years in prison for three Facebook posts that were considered to be a violation of the law.

The sentence comes on top of another four-year term handed to him last year for the content of a speech he gave in 2020.