The newspaper said officials from these countries admitted they voted with the pro-whaling grouping because of the aid they received from Japan, or because they were given cash or call-girls.
The sting was conducted ahead of a meeting of the IWC in Morocco this month at which Japan is expected to push its case for allowing more whaling.
Undercover reporters posing as representatives of a Swiss billionaire conservationist approached officials from pro-whaling countries and offered them an aid package to switch their vote.
During their negotiations, the officials revealed the reasons for supporting Japan.
The IWC proposal would allow Japan, Norway and Iceland to hunt a set quota of whales despite the 1986 moratorium on commercial whaling.
Japan gets around the ban on commercial whaling by arguing that it harpoons hundreds of whales each year for scientific research.
Meanwhile the south Pacific island nation of Palau has ended its support for Japan's scientific whaling programme, switching instead to a proposed annual whale catch quota.
The proposal will be voted on at the Morocco meet.
The quota system, which could halve the 1,000-plus whales Japan would harvest annually over the next five years, will only be implemented by unanimous approval of the IWC's 88 members, including Japan.
Palau relies heavily on Japan for aid, its biggest benefactor after the United States.
The IWC is separately trying to resolve long-running disputes over the hunting of whales by several countries despite a 25-year-old moratorium on hunts.