Two key members of Prime Minister Shinzo Abe's Cabinet have admitted to filling out blank receipts as part of their expense claims, raising questions about their ethics and the efficacy of a political funds control law.

Chief Cabinet Secretary Yoshihide Suga and Defense Minister Tomomi Inada told an Upper House budget committee session on Thursday that they had for the past few years had office staff fill out the blank receipts they received after attending fellow Liberal Democratic Party lawmakers' fundraising parties and paying party fees.

According to Akira Koike, a Japanese Communist Party lawmaker who grilled the pair during the session, the expenses declared on such receipts between 2012 and 2014 amounted to ¥18.75 million and ¥5.2 million for Suga and Inada, respectively.