In a radio interview, Singson said the report was baseless and accused the camp of deposed President Joseph Estrada of spreading lies to discredit him.
He said Napolcom vice chairman Rogelio Pureza "maybe took a bribe" to put out the report.
Singson is the government’s main witness against Estrada, who is on trial before the Sandiganbayan for plunder and other corruption charges.
Estrada was ousted in 2001 and was replaced by then vice president Gloria Arroyo.
A Sept. 16 memorandum of Pureza to Interior Secretary and concurrent commission chairman Joey Lina Jr. said Singson is among about a dozen people behind illegal gambling operations, including the popular jueteng lotteries, in six provincial regions.
The Office of the Ombudsman said it will investigate the allegation.
However, Pureza said his report was "merely a transmittal of unverified and raw information gathered from various sources".
He said the information still has to be verified because it may have come from "enemies of some officials."
"The reports picked up by the media allegedly containing a list of jueteng lords that include the name of Singson has absolutely no basis," Pureza said in a press statement yesterday.
"The memorandum is a regular internal communication which is part of an update report on the commission’s on-going national anti-gambling campaign."
In 2000, Singson accused Estrada of running a protection racket for illegal gambling operators while he was President and using the proceeds to acquire mansions for himself and his string of mistresses.
He said he acted as Estrada’s bagman, collecting millions in bribes on his behalf. Estrada lawyers say Singson was the one who ran the protection racket.
Estrada was ousted in January 2001 by a military-backed massive protest after his impeachment trial ended in a farce.
Estrada, who still remains hugely popular among the poor, has steadfastly denied any wrongdoing and maintains he was illegally ousted from the presidency.
He refused to enter pleas during separate arraignments on the charges. The Sandiganbayan entered innocent pleas on his behalf.
Plunder or massive corruption is a crime theoretically punishable by death. Officials have said Estrada, if convicted, would likely be spared.
The government granted Singson immunity in exchange for his testimony against his estranged drinking buddy.
Acting Ombudsman Margarito Gervacio said Singson is covered by his immunity if his reported involvement in illegal gambling occurred when he was still governor.
Pureza’s report said Singson allegedly operated masiao – a lottery similar to jueteng – in the Eastern Visayas region.
"Masiao and ‘two digits’ are still the most common forms of gambling in the province of Leyte and some parts of the region. These were operated by two groups; one has been identified with Wacky Salud/Chavit Singson group and the other is identified with the group of Mitch Ortega of Cebu," Pureza’s report said. — With Marichu Villanueva