Japan's ruling party will vote September 14 on a replacement for Prime Minister Shinzo Abe, the country's longest-serving premier, who is resigning for health reasons, local media reported on Tuesday.

An official announcement from the Liberal Democratic Party is expected either later Tuesday or Wednesday, but a decision on a limited vote that will exclude rank-and-file members has already been approved.

Candidates will be required to register their run on September 8, with campaigning immediately after.

The LDP election is for the party leadership, with a parliamentary vote for prime minister following on September 16, reports say. Opposition candidates are expected to stand, but the vote is seen largely as a formality that will endorse the ruling party's new leader.

A leadership contest would usually poll ruling party lawmakers and LDP members nationwide.

But the LDP has opted for a constrained format, polling only its MPs and three representatives from each of the country's 47 prefectures.

The decision has elicited some criticism, but party officials said it would take too long to organise a broader vote.

"The prime minister is sick, his illness has re-emerged. He is working hard under very difficult conditions," Shunichi Suzuki, chairman of the party's general council, told reporters on Tuesday.

"If we were to hold a full-scale election, it would take two months. With the prime minister's health, two months is very long. We can't take that long," Suzuki said.

Several candidates have already emerged for the race, with chief cabinet secretary Yoshihide Suga considered the presumptive frontrunner.

Suga, 71, has held his powerful post for years -- coordinating policy among ministries and agencies, and serving as the effective face of the government as its chief spokesman.

Considered a pragmatist without strong ideological positions, Suga is a close Abe advisor who encouraged the prime minister to run again after a disastrous first term in office ended after just a year in 2007.

Other candidates are expected to include former defence minister Shigeru Ishiba, who polls well with the general public but is less popular among party members.

A military expert, Ishiba once left the LDP, spending time as both an independent and briefly joining another party.

Many within the LDP have not forgiven the political dalliance.

LDP policy chief Fumio Kishida, a former foreign minister, is also expected to stand.

He is often described as Abe's favoured successor, but his limited public profile and low-key persona are likely to pose obstacles as he launches a challenge to the likes of Suga.

Abe announced his shock decision to step down on August 28, explaining he was suffering a recurrence of ulcerative colitis, a condition that has plagued him for years.

He is Japan's longest-serving prime minister, but has seen his government's approval ratings sink in recent months with the public unimpressed by his handling of the coronavirus outbreak.