Rice prices in Japan soared 99.2 percent in June year on year, official data showed on Friday, piling further pressure on Prime Minister Shigeru Ishiba ahead of elections this weekend.

Public support for Ishiba's administration has tumbled to its lowest level since he took office last year at the head of a Liberal Democratic Party (LDP) coaltion, partly because of frustration over the cost of living.

One of the main sources of anger has been inflation and in particular the surging cost of rice, as well as scandals within the ruling party.

The price of the grain already rocketed 101 percent year on year in May, having jumped 98.4 percent in April and more than 92.5 percent in March.

Overall, Japan's core inflation rate slowed to 3.3 percent last month from 3.7 percent in May, data from the internal affairs ministry showed.

The reading, which excludes volatile fresh food prices, was slightly below market expectations of 3.4 percent.

Excluding energy and also fresh food, consumer prices rose 3.4 percent, compared with 3.3 percent in May.

Opinion polls ahead of Sunday's election suggest the ruling coalition may lose its majority in the upper house.

This could force Ishiba to resign after less than a year in office.

In October, his coalition was deprived of a majority in the powerful lower house.

It was the worst election result in 15 years for the LDP, which has governed Japan almost continuously since 1955. (AFP)