Former Democrat party leader Chuan Leekpai described as “unbelievable” on Tuesday that 16 Democrat MPs voted in favour of a Pheu Thai prime ministerial candidate, despite the long rivalry between the parties.

The MPs in question did not follow the party’s resolution, to abstain in the vote, Chuan said, adding that he had obtained the party’s permission to vote against Srettha Thavisin, the Pheu Thai party’s successful candidate.

Former real estate tycoon Srettha was chosen by parliament as the 30th prime minister of Thailand on Tuesday.

“The Democrat party has fought with Pheu Thai in the political field for many years, particularly in the southern region. The governments of Thaksin and Yingluck Shinawatra in the past had unfairly cut budget allocations for the south many times.”

Of the 16 Democrat MPs who voted for Srettha, 12 represent constituencies in southern provinces.

Chuan, a former prime minister, said one of the 16 MPs “threatened” him to get him to vote in line with the party’s resolution. “He said, if I did not follow the resolution, I would have to quit the party. It was, however, him who voted against the resolution,” said Chuan.