The Diet on Wednesday passed a controversial bill, sponsored by Prime Minister Shinzo Abe's ruling party, to increase the number of seats in the Upper House for the first time in nearly five decades in pursuit of making the electoral system fairer.

The approval of the bill by a majority vote in the Lower House, which is controlled by the ruling coalition, will add six seats to the House of Councillors amid efforts to narrow what is known as the vote-value disparity.

Opposition parties, however, have criticized the Liberal Democratic Party for prioritizing "partisan interests" above rectifying the gap in the weight of a single vote between urban and rural constituencies.

Despite strong objections from opposition lawmakers, the bill cleared the House of Councillors last week and a Lower House panel on Tuesday.

Despite Japan's shifting demographics and calls from the public to trim the number of parliamentarians to save taxpayer money, the increase, opposition representatives claim, is intended to "bail out" incumbent LDP lawmakers who will be unable to run for their rural constituencies in the next Upper House election.

The passage of the bill by the more powerful House of Representatives will increase the number of seats in the Upper House by six from the current 242 in the run-up to the chamber's next election, set for the summer of next year.