NHK has learned that certain secretaries working for lawmakers belonging to Japan's main ruling party's largest faction said they used to include kickbacks as income in political fund reports, but stopped doing so following the faction's instructions.

Tokyo prosecutors are believed to be investigating kickbacks given to the offices of most members of the Liberal Democratic Party's largest faction previously led by late Prime Minister Abe Shinzo.

The faction is suspected of giving the money to offices of lawmakers who exceeded quotas for sales of fundraising event tickets, without reporting the revenue.

There are suspicions that the Abe faction systematically treated the unreported money as slush funds.

Kickbacks from Abe faction fundraisers over the past five years are believed to total about 500 million yen, or around 3.5 million dollars.

Earlier, it was found that multiple secretaries working for Abe faction lawmakers told prosecutors that they had received kickbacks from the faction in cash, and the group told them not to report the money in their income and expenditure reports.

Other secretaries are said to have explained that the faction informed them that there was no need to report the kickbacks because they were given to individual lawmakers as political activity funds, which do not require reporting.

The prosecutors are said to be questioning on a voluntary basis lawmakers who received significant kickbacks.