A day after announcing he was stepping down as Japanese Prime Minister, Shinzo Abe has been admitted to hospital suffering from extreme exhaustion and stress.

Abe had been facing growing calls to quit after his party lost Upper House elections in July. There has been speculation that health problems were partly behind his decision to step down.

The ruling Liberal Democratic Party will meet next week to choose a new party leader, who will automatically become Japan?s new Prime Minister. Meanwhile, speculation has been mounting in Tokyo that former Japanese prime minister Junichiro Koizumi will come out of retirement after the sudden resignation of Shinzo Abe.Japan?s finance minister became the first to launch a bid to lead the country on Thursday as the ruling party scrambled to avoid a policy vacuum after Prime Minister Shinzo Abe?s shock resignation. Abe?s year in office has been marked by scandals involving cabinet members and a disastrous election defeat in July.

But the conservative leader?s decision to step down now sparked criticism and concern that the ensuing confusion could stall vital decisions on policies such as tax and fiscal reform.

Finance Minister Fukushiro Nukaga, 63, said he would enter the race for president of the ruling Liberal Democratic Party (LDP) and hence premier. A former defence and economics minister, Nukaga has twice had to step down over scandals but was one of several veterans tapped by Abe for a revamped cabinet just last month. Abe had said he was quitting over a stalemate in parliament but officials said health problems were also a factor. His doctor told a news conference he was suffering from a gastrointestinal disorder and would likely be hospitalised for three or four days.