By Linda Sieg

TOKYO (Reuters) - Japan is moving to adopt a law allowing its octogenarian Emperor Akihito to abdicate but many touchy topics, such as his title and duties, remain to be settled before the monarch can retire in a step unprecedented for two centuries.

Japanese law does not currently allow an emperor to give up the throne, but Akihito, 83, who has had heart surgery and prostate cancer treatment, said in rare public remarks last August he feared age might make it hard to fulfill his duties.

A panel of experts is expected on Monday to indicate a preference for a special law to allow Emperor Akihito to retire, most probably by the end of 2018.

Officials are looking at ancient precedents, since the last time an emperor abdicated was in 1817.

"Japan is in uncharted territory except for historians," said Colin Jones, a professor at Doshisha University Law School.

Emperor Akihito may also look to examples of foreign monarchs of his generation, such as Dutch Queen Beatrix and Belgian King Albert II, or even Pope Benedict. All three retired in 2013.

Beatrix resumed her former title of princess and remained active, while Albert retained his title, though mostly staying out of public view. Benedict, as "pope emeritus", moved to a monastery inside the Vatican walls.

"The emperor himself was friends with Queen Beatrix and King Albert since his days as crown prince," said Naotaka Kimizuka, an expert in European monarchies at Kanto Gakuin University. "I think he will probably consider them as reference points."

Besides the emperor's title, duties and residence after he retires, the government also has to fix the name and date for the commencement of the new "imperial era" that will be ushered in by his successor, under Japan's unique calendar.

Once considered divine, Japan's emperor, or "tenno", is defined in the post-war constitution as a "symbol of the state and of the unity of the people". He has no political power.

Nonetheless, the title he is to be accorded after abdication is a touchy matter. History suggests Akihito should get the title "joko", meaning "retired emperor".

But some experts say the term echoes ancient episodes when the former emperor retained power and clashed with his successor. They prefer terms such as "zen tenno" or "moto tenno", meaning former, or previous, emperor respectively.

Emperor Akihito's duties include Shinto religious ceremonies and constitutionally-defined tasks, such as the opening of parliament. These will be taken on by his successor, Crown Prince Naruhito.

The hallmark of the popular monarch's reign has been travels with Empress Michiko, such as visits to domestic disaster sites to cheer survivors, and overseas, to soothe the wounds of a war fought in the name of his father, Emperor Hirohito.

Given Emperor Akihito's belief that his public activities are central to his symbolic role, he may well let Crown Prince Naruhito take center stage. [http://tmsnrt.rs/2b7dWHn]

"There is concern over splitting the message, splitting the symbol," if Emperor Akihito stays too active, Doshisha's Jones said.

The emperor's younger son, Prince Akishino, has suggested his parents would enjoy activities such as music and the marine biology research in which Akihito specializes.

A pay cut does seem in store for the imperial couple, who do not have a large personal fortune, unlike the British royals.

If they retreat to the shadows, they will probably get an annual allowance roughly equivalent to the $400,000 received by Emperor Akihito's younger brother and his wife - or about a fifth what they get now, Kimizuka said.

Residence options for the couple include the Togu Palace, now home to the crown prince, and the Fukiage Omiya Palace, where Emperor Hirohito once lived, both within the spacious palace grounds in Tokyo.

The ancient imperial capital of Kyoto also cannot be ruled out, some experts said.

Once Emperor Akihito steps down, a new "imperial era" will begin, replacing the current "Heisei", meaning "achieving peace", which began on Jan. 8, 1989, the day he took the throne.

Although Japan uses the Western-style Gregorian calendar, it has also kept its ancient imperial-era system - "nengo" or "gengo" - in which a new emperor ushers in a new era.

The government might break with precedent and announce the name of the new era months before Emperor Akihito retires, to allow time for the switch to be made in official forms, calendars and computer programs, media have said.

(Editing by Clarence Fernandez)