Japanese medallists of the Rio 2016 Olympic Games attend a flag-raising ceremony.

Tokyo could be forced to downsize or relocate three venues for the 2020 Olympic Games in drastic cost-slashing measures, the city's new governor said Thursday.

A group of academics and business leaders submitted a proposal to governor Yuriko Koike which included plans to overhaul the swimming, volleyball and kayak facilities, currently estimated at some $1.5 billion.

It comes after a series of embarrassing problems for Tokyo organisers, including the scrapping of plans for the Olympic stadium and a French probe into alleged corruption.

Koike swept into office in July, immediately ordering officials to rein in ballooning Olympics costs, which one expert predicted could surpass 3 trillion yen ($29.5 billion).

"I take this report seriously," Koike told reporters.

"After discussing it with the people in charge (of the Games), I will make a decision as soon as possible.

"We can't pass this burden on to Tokyoites."

Professor Shinichi Ueyama of Keio University, who was on the panel, warned the initial Games cost of 734 billion yen would skyrocket past 3 trillion yen.

"Surprisingly there is no chief executive, and no chief financial officer" to control the budget, he said.

The 97-page report noted that the 49 billion yen Sea Forest Waterway rowing and kayaking venue was opposed by some athletes over access and wind concerns, and that there was "no concrete financial plan for its use after the Olympics".

An existing venue in Miyagi prefecture, some 400 kilometres (250 miles) north of Tokyo, could be used as a potential alternative location, it said.

Experts also proposed using an existing swimming pool in Tokyo rather than build a new Olympic aquatics centre at an estimated cost of 68.3 billion yen.

"In theory there must be a number of alternative venues other than a (new) aquatics centre. It is a big problem that no consideration 

Source : AFP