Faizer Mustapha, a President's Counsel and deputy minister of investment, switched allegiance before next week's presidential election in which Maithripala Sirisena is challenging incumbent Mahinda Rajapakse.
Sirisena, who himself defected last month to take on Rajapakse, has vowed to withdraw generous tax concessions granted to foreign-funded casinos.
Mustapha has been a strong proponent of three mega "integrated resorts", a government euphemism for plush casinos, and has been the public face of Rajapakse's controversial policy both in and outside parliament.
Mustapha's defection brought to 24 the number of ruling party lawmakers who have joined the opposition to Rajapakse, who is South Asia's longest serving leader.
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Rajapakse called the election two years ahead of schedule before his popularity waned further after his party suffered a 21-point decline in September local polls.
Parties representing Tamils, the country's largest minority ethnic community, as well as the group representing Muslims, the second largest minority, have supported Sirisena.
After crushing Tamil guerrillas and ending a 37-year separatist war in May 2009, Rajapakse abolished the two-term limit on the presidency, tightened his grip on the police, the judiciary and the civil service and also legalised casinos and other forms of gambling.
Sri Lanka's influential Buddhist clergy had opposed casinos, including one with investment from Australia's gambling mogul James Packer.
None of the proposed gambling resorts have opened in Colombo yet but several local casinos have been in operation for decades, exploiting legal loopholes.