Future Forward Party (FFP) leader Thanathorn Juangroongruangkit has demanded an apology from Nation TV for airing a doctored clip appearing to be a conversation he had with former prime minister Thaksin Shinawatra.

"I am demanding Nation [TV] apologise to me and the people who heard this content," he told Nation TV on Wednesday.

His demand followed the decision by Nation TV to broadcast a recording of a conversation it suggested was between the FFP leader and Thaksin on its Khao Khon Khon Nation programme on Tuesday night.

"I want you to listen to a clip going viral on social media. I am not sure whose voices they are," Kanok Ratwongsakul, co-anchor of the news programme, said before putting the clip to air.

Mr Kanok did not mention Mr Thanathorn or Thaksin by name, but the graphic accompanying it appeared to represent them and the voices were similar.

Netizens closely examined the clip and concluded the conversation was doctored. The voices of Thaksin were spliced, originating from at least two separate occasions. One was from his meeting with students at Samsenwittayalai School and another was an interview with BBC Thai. 

Nation TV interviewed Mr Thanathorn by phone on Wednesday in another programme, and he confirmed that the clip was faked.

"The decision of Nation to air that clip showed the ethical standard of the Nation is on the decline," he told the two hosts.

The doctored clip and Nation TV has become the talk of the town on social media and topped the Twitter hashtag in Thailand. Netizens expressed their disappointment with Nation TV for its decision to show the clip on Tuesday, and with the programme on Wednesday for trying to press him to say whether he had talked to Thaksin.

Thaksin has not reacted to the clip, but his son Panthongtae tweeted on Wednesday with a Thai hashtag meaning shame in English.