His stance has politicians and academics warning about a conflict of interest and a breach of political etiquette.
The term outsider prime minister refers to a nominee not on the list of political parties and who is voted in by a majority in both the House and Senate.
Democrat leader Abhisit Vejjajiva on Friday said even though Mr Somkid has the political right and that, as deputy prime minister, he should be careful and refrain from exploiting his authority for political leverage.
The principle of good governance should be upheld and a conflict of interest should be avoided otherwise the much-anticipated national reforms would be stalled, he added.
Mr Somkid recently expressed his support for Gen Prayut after reporters asked if he was ready to continue to work in the government for the next five years after the poll, expected in February next year.
The deputy premier also dropped a hint that a new party is being formed to support Gen Prayut and told reporters to seek answers from Industry Minister Uttama Savanayana and Commerce Minister Sontirat Sontijirawong.
The remarks coincided with media speculation after a political group applied to register the Pracharath Party with the Election Commission (EC), with Mr Uttama speculated to be the new party's leader and Mr Sontirat its secretary-general.
Mr Abhisit played down reports about defections after some party members including Natapol Teepsuwan were said to seek a meeting with Mr Somkid, saying it was normal for a new party to recruit members.
However, Mr Abhisit said he has not held any discussions about the issue with Mr Natapol.
Democrat deputy leader Sathit Pitutecha warned Mr Somkid to consider maintaining political impartiality.
According to Mr Sathit, such remarks would raise suspicions about fair use of state budget and resources to favour a political party or group in an election campaign.
"It's okay to show support, but when Gen Prayut makes it clear he's in [a political contest], it's not appropriate," Mr Sathit said.
Academics looked at Mr Somkid's move as an attempt to mobilise support for Gen Prayut who was widely speculated to return to power as an "outsider prime minister".
They said Mr Somkid's support for Gen Prayut did not come as a surprise.
Thitipol Phakdiwanich, dean of political science faculty of Ubon Ratchathani University, said the regime has rolled out policies and projects with the intent to win votes for Gen Prayut.
"It didn't surprise me. The government has done things to facilitate Gen Prayut's return as the prime minister," he said.
He said Mr Somkid's remarks could be seen as trying to draw support from the business community and the move could be triggered by the emergence of the Future Forward Party led by Thanathorn Juangroongruangkit, the 39-year-old executive vice-president of the Thai Summit Group, which has drawn wide public attention.
"Mr Somkid has the image of a successful businessman and is widely accepted in the business sector. Mr Thanathorn's party has generated so much hype and that's why Mr Somkid had to act," he said.
Chaiyant Ratchakul, a political scientist at Phayao University, urged Mr Somkid to observe political etiquette, saying the remark could be perceived as a conflict of interest because he is a deputy prime minister.
He also questioned the transparency of government spending schemes, saying the public would be sceptical if state funds were spent to influence an election outcome.
Nisit Sinthuprai, a member of the Pheu Thai Party and a red-shirt supporter, expressed concerns about possible abuse of authority to create conditions favourable for certain individuals or political parties.
"It is not a problem to set up a party to support a particular person. But I'm concerned that Gen Prayut, who is the prime minister and the head of the National Council for Peace and Order might abuse state mechanisms for political gain," he said.
He pointed out that the constitution and other laws have apparently been designed to create the opportunity for Gen Prayut to come back as prime minister and the regime has advantages over other parties.
Asked if he was concerned some party members would defect from the pro-Prayut party, he said as long as an election was fair and transparent, defections should not be a cause for concern.
The Pheu Thai member also praised the Democrat Party leader for taking a stance on the outsider prime minister issue, referring to the latter's remark that those who wanted to support Gen Prayut should join other parties.
Amnuay Khlangpha, former Pheu Thai MP for Lop Buri, brushed aside concerns over Mr Somkid and the new party supporting Gen Prayut, saying the political allies were loosely tied by common interests.
He said if Mr Somkid steps down from the post, he and his political outfit will lose support.
"That party can't compete with others like Pheu Thai, the Democrat Party or Bhumjaithai Party which have strong support bases," he said.
Meanwhile, Wirat Kalayasiri, chief of the Democrat Party's legal team, urged former prime minister Thaksin Shinawatra to follow in his sister's footsteps and wash his hands of politics.
He was referring to a recent announcement from Somchai Wongsawat that his wife, Yaowapa, had left Pheu Thai when she did not show up to confirm her party membership status.
Mr Wirat said even though Thaksin maintained he had nothing to do with Pheu Thai affairs, the fugitive prime minister was simply paying lip service.
In a separate development, the Office of Election Commission said it had approved the registration for another four groups. A total of 98 groups applied to register new political parties with the EC in March.
See original story here.