Thailand's ruling coalition has scored another win, just days after getting a budget past its first reading through Parliament.

A member of the ruling coalition yesterday won a by-election in Nakhon Pathom province, an hour's drive from Bangkok, seizing the constituency from an opposition party. With 90 per cent of the vote counted, Mr Padermchai Sasomsap, a veteran politician from the Chart Thai Pattana Party, was in the lead with 38,765 votes.

The opposition Future Forward Party, which had grabbed the constituency with 34,164 votes in the March general election, could manage only 28,882 votes yesterday.

"Today's vote shows the spirit of unity. We're all family in Nakhorn Pathom, wherever we come from," said Mr Padermchai, 70, who was a veteran MP for the constituency. His family has deep connections to the area.

He last won the seat in 2011 when he campaigned under the banner of the Pheu Thai Party, which is closely linked to the Shinawatra clan. He won that year by an overwhelming margin - garnering 48,000 votes - but in the March election he ended up in fourth place with 12,279 votes while campaigning as a Chart Thai Pattana candidate.

He was labour minister from 2011 to 2013 during the Yingluck Shinawatra administration.

The by-election was triggered after the previous Future Forward MP, Mrs Chumpita Juntarakajorn, 55, had to resign because of ill health. Her husband, Piratachot Juntarakajorn, 59, had stepped in to try to take her place on the same party ticket.

"The results are not surprising. Everyone focused only on Future Forward. But this is not like the March election where the momentum the party had set was at its peak. Now, it's simply a decision of the locals," said political science professor Attasit Pankaew from Thammasat University in Bangkok.

With heavyweights like Pheu Thai, a party founded by allies of former premier Thaksin Shinawatra, and the pro-military Palang Pracharat Party sitting out the vote, the by-election was a proxy contest between the coalition government and opposition front.

Many had called into question the strength of the government in Parliament as the ruling coalition, comprising 16 parties, had only seven more seats than its rival. The ruling coalition controls 250 seats in the House of Representatives against the opposition's 243.

The government managed to secure the passage of the first reading of its budget Bill by 251 votes late last Saturday after a three-day session. The opposition bloc abstained. The budget will return to Parliament in January for the second and third readings.

Mr Thanathorn Juangroongruangkit, leader of the anti-military Future Forward Party, had earlier this month urged voters in the by-election to send a signal to the government that they have had enough of it.

Mr Thanathorn is now facing the prospect of being disqualified as an MP and his party is also in danger of being dissolved.

Mr Prayut Chan-o-cha took power in a May 2014 military coup and became Prime Minister three months later.

He was nominated for the prime minister's post by the pro-military coalition led by the Palang Pracharat after the March election, a choice rubber-stamped by the Senate.