Japan's Prime Minister Shigeru Ishiba hit out at "biased" media reports related to a party scandal, as a fresh survey suggested that his ruling coalition could fall short of a majority in elections on Sunday.

This would be the worst result for the Liberal Democratic Party (LDP) – which has governed Japan for almost all of the past seven decades – since it last lost power from 2009 to 2012, as well as a major blow to Ishiba himself.

Adding to Ishiba's woes is the continuing fallout from a slush fund scandal within the LDP that angered voters and helped torpedo his predecessor, Fumio Kishida.

Ishiba promised to not actively support LDP politicians caught up in the scandal running in the election, although they are still standing.

But according to media reports, the party has still provided US$132,000 each to district offices headed by these figures.

"It is truly frustrating that such reports come out at a time like this," Ishiba said in a campaign speech on Thursday. "Those candidates will not use the money."

"We cannot be defeated by those with biased views," he added.

Opposition leaders pounced on Ishiba's comments, including Yoshihiko Noda, a popular former prime minister who heads the main opposition Constitutional Democratic Party (CDP), the second-biggest in parliament.

"However you look at it, it is cash to secretly endorse them. Mr Ishiba is making excuses that no one understands," Noda, 67, said in a campaign speech.

"He is angry with these reports? What are you saying? It is the Japanese people who are angry." (AFP)