Tokyo Governor Yuriko Koike speaks during an interview with AFP (AFP Photo/Behrouz MEHRI)

Tokyo (AFP) - Tokyo's governor has warned that the bill for the city's 2020 Olympics could yet spiral out of control despite exhaustive efforts to rein in runaway costs.

Governor Yuriko Koike said there was still a danger of costs reaching an eye-watering 3 trillion yen ($26 billion) -- four times more than initially planned -- unless Games organisers keep a tight rein on the purse strings.

"We still haven't got a final figure for how much it will cost," she told AFP in an interview Tuesday.

"But if there is no governance within the organising committee, it could be that expensive.

"That's a question for them," added the governor, who took office last August after a landslide election victory.

"In short, it's a question of leadership and good management."

Concerns over soaring costs have cast a shadow over Tokyo's Olympic preparations and fears were heightened after a panel of experts predicted the overall budget could exceed 3 trillion yen without drastic cuts.

Koike proposed a review of three costly venues -- for rowing, swimming and volleyball -- but subsequently agreed to keep all three at their planned Tokyo sites.

"I don't think we are on different wavelengths," she said, rejecting any suggestion of discord between 2020 organisers and city and central government officials.

"Everyone is cooperating," added the former TV newscaster.

"At one point the Sea Forest (rowing) venue was going to cost $1.1 billion but we have more than halved that. Tokyo is building six venues and we have made significant reductions to the costs."

Tongue planted firmly in cheek, Tokyo's first female governor joked: "I just show the differences (in opinion) as much as possible to make it news!"

- Golf controversy -

In its bid, Tokyo estimated costs of $7 billion and projected an economic windfall of 3 trillion yen, but Tokyo organisers are under pressure to balance the books after ballooning costs at recent Olympics, notably the $50 billion price tag for the 2014 Winter Games in Sochi.

The International Olympic Committee (IOC) is under pressure to reduce costs to keep cities interested in bidding for future Games.

"It's just not sustainable to spend vast sums of money on the Olympics," said Koike, 64, who previously served as Japan's defence minister.

"Tokyo will be a test case for the IOC."

Koike recently criticised the Tokyo Olympic golf venue for not accepting women as full members, leaving the private club's hosting rights in the balance.

"Kasumigaseki Country Club not admitting full women members conflicts with Olympic Charter," she said, referring to the club in Saitama prefecture, north of Tokyo.

"We will continue to monitor the situation and see how they react."

A long-time parliamentarian, the Cairo University graduate and fluent Arabic speaker is seen as one of Japan's most popular politicians.

Tipped as a possible future prime minister, Koike says she leads a "normal life" at home, doing laundry and going for walks with her pet Yorkshire terrier.

But as she continues her fight against Olympic over-spending, the political reformer insisted she was committed to the task in hand.

"I'm a new governor and it's my duty to deliberate on every venue, one by one," Koike said.

"Tokyo has a heavy responsibility to make the 2020 Games a success."