The Civil Court on Thursday ordered Banpu to pay over 31 billion baht in damages to Mr Siva for what was characterised as "deception" on the part of the listed company over a partnership to develop a coal mining and power project in Hongsa, Laos.
Theerapun Petchsuwan, a lawyer representing Mr Siva, said his client will file an appeal despite his victory, pressing for higher compensation.
Mr Siva had originally sought damages of 63.5 billion baht from Banpu.
Mr Theerapun told the Bangkok Post that Banpu could land in more trouble due to a separate suit between Mr Siva and the Lao government.
Mr Siva is seeking payment of US$71 million (2.19 billion baht) from Laos for breaking the original contract held by his company, Thai-Lao Lignite, for the Hongsa power project.
Mr Theerapun claimed to have documents that state Banpu had agreed to compensate the Lao government for any legal costs related to the Thai-Lao Lignite lawsuit.
Thai-Lao Lignite won a concession from the Lao government in 1994 to develop a coal-fired power plant for electricity sales to Thailand.
In 2004, Banpu entered into a joint venture with Thai-Lao Lignite Co for the project.
But the Lao government in 2006 scrapped the project, citing a lack of development. Later, Thai energy authorities revised upwards its power demand plans, leading Laos to reopen bids for the Hongsa concession, which was won by Banpu in 2006.
In the Thai Civil Court case, Mr Siva accused Banpu of using information gained from their joint venture and "misinforming" the Lao government to cancel Thai-Lao Lignite's original concession. The Civil Court awarded Mr Siva damages of 4 billion baht and additional compensation for opportunity losses of 860 million baht a year from 2015-27 and 1.38 billion baht a year from 2028-39.
Mr Theerapun said Mr Siva had sued three companies connected to the case: Banpu Plc, Banpu Power and Banpu International. The damage claim was 4 billion baht for the feasibility study, and 59.5 billion baht for revenue losses due to the cancellation of the Thai-Lao Lignite concession.
Mr Theerapun said under the original contract, Thai-Lao would be paid for coal used to generate electricity from the Hongsa project, equal to another $577.5 million based on demand estimates of 350 million tonnes over the lifetime of the concession at a cost of $1.65 a tonne.
"[Mr Siva] is likely to appeal to cover this loss as well as for other opportunity losses. We will know the exact amount once the full [court] ruling is completed. After that, we will submit a petition, which may be in November or December," Mr Theerapun said.
He said Thai-Lao Lignite would also appeal the court's decision to dismiss lawsuits filed against three directors of Banpu: president and chief executive Chanin Vongkusolkit, Ongart Auapinyakul and Chanchai Jivacate.
Mr Theerapun said Mr Siva has debts of more than 10 billion baht borrowed for the Hongsa project and was now essentially bankrupt due to the cancellation of the concession.
"According to the feasibility study, the Hongsa power plant will generate profits of around 100 billion baht over the term of the project. It's only fair that compensation is paid to Thai-Lao Lignite, the former owner of the project," Mr Theerapun said.
The case between Thai-Lao Lignite and the Lao government has been in arbitration since 2007.
James Berger, a lawyer with Paul, Hastings, Janofsky & Walker, a law firm representing Mr Siva in the case, said the United Nations Commission on International Trade Law had ruled that Laos had to pay $57.2 million to Thai-Lao Lignite for breach of contract.
The Lao government has refused to pay the award, leading Thai-Lao Lignite to file for enforcement of the award in courts in London, Paris and New York.
Mr Berger said in an email interview that a French court confirmed the arbitration ruling in 2010, while a US court affirmed the ruling on appeal earlier this year. With interest, the total award is now worth over $71 million.
Both Mr Berger and Mr Theerapun confirmed that Banpu ultimately could be held responsible for the ruling.
Executives of Banpu could not be reached for comment. The company is expected to make an announcement on the case today.
In a statement to the Stock Exchange of Thailand (SET) last week, Banpu chief financial officer Somruedee Chaimongkol denied any wrongdoing and said the company would appeal against the verdict.
In an interview with the Bangkok Post last month, Mr Chanin insisted the Hongsa project would not be affected by the litigation. He said that Thai-Lao Lignite had invited Banpu to participate in the project.
Mr Chanin denied any role that may have led the Lao government to terminate the Thai-Lao Lignite concession, and insisted that Banpu had won the new bid fairly.
Shares of Banpu fell 38 baht or 8.6% to 404 baht in trade on the SET on Friday.
Analysts said the judgement could wipe out more than two years of the company's profits, as a legal reserve against the award is set up pending a court appeal.