TOKYO (Kyodo) -- The office of former Prime Minister Fumio Kishida handed out gift vouchers to parliamentary vice ministers during his premiership in 2022, ruling party sources said Wednesday, a new revelation that puts the spotlight on gift-giving by political leaders.

"We always followed the law," Kishida's office told Kyodo News. He was prime minister between 2021 and 2024, before handing over to incumbent Shigeru Ishiba last year.

Ishiba is now in the hot seat over his own distribution of gift certificates worth 100,000 yen ($670) each to 15 new lower house lawmakers of the Liberal Democratic Party to express his "appreciation" to them.

According to the sources, Kishida's staff delivered vouchers worth 100,000 yen each to LDP lawmakers who at the time served as parliamentary vice ministers before or after a meeting at the prime minister's official residence in 2022.

The recent revelations came as the LDP is still reeling from a slush funds scandal that was one of the factors causing the ruling coalition to lose majority control of the powerful House of Representatives in October, with Ishiba left in charge of a minority government. The slush funds scandal emerged when Kishida was prime minister.

Opposition parties are turning up the heat on the scandal-mired LDP, with a member of the Constitutional Democratic Party of Japan demanding that Kishida answer questions in parliament.

Ishiba, who has defended his own voucher-giving as legal while acknowledging it was out of synch with public sentiment, declined to comment on Kishida's case during a parliamentary session on Wednesday.

The premier himself has been facing calls by opposition party leaders for him to testify before the political ethics committee of the Diet, though he has appeared reluctant to do so. Under Japanese law, donations by individuals to politicians for the purpose of political activities are banned.

The office of Taro Aso, who was prime minister between 2008 and 2009, declined to give a clear answer when asked by Kyodo News whether gift vouchers were distributed while he was in office.

"We handled it appropriately, taking into consideration the nature of meetings with parliamentarians," the office said.

Yoshihiko Noda, who served as premier between 2011 and 2012 and now serves as CDPJ leader, told reporters he had "never" given out vouchers.

"This looks to be an issue with the LDP's political culture. It is shaping up to be a deep-rooted structural one," Noda said.

The recent revelations have cast a shadow over parliamentary deliberations on a draft state budget for the next fiscal year from April to help ease the pain of inflation felt by households. They also come ahead of an election this summer for the House of Councillors, the upper chamber currently controlled by the ruling coalition.

An upper house member of the LDP said Sunday it was a "standard practice" followed by past prime ministers to distribute gift vouchers. But he later retracted the comment, saying it was not based on facts.