By Elaine Lies and Chris Gallagher

TOKYO (Reuters) - A Tokyo panel on Thursday urged changes to three venues for the 2020 Olympics in the face of ballooning costs but organizers say such adjustments may prove difficult to instigate in time for the Games.

Tokyo touted access to a $4.5 billion war chest when it beat Madrid and Istanbul in its successful 2013 bid for the Summer Games but recently elected governor Yuriko Koike campaigned on reining in ballooning costs and ordered a review of expenses.

The proposed changes, which could include moving rowing and canoeing some 400 km (250 miles) from Tokyo, would require the approval of the International Olympic Committee (IOC) and each sport's international federation.

They are the latest in a series of embarrassing setbacks and broken promises for organizers, who won the bid largely on Japan's reputation for efficiency.

Tokyo's bid proposal, for example, said some 85 percent of venues would be within 8 km (5 miles) of the Olympic Village in central Tokyo.

According to a preliminary report, released on the same day as an executive board meeting of the Tokyo 2020 organizing committee, overall costs could surge to more than four times original estimates of 734 billion yen ($7.24 billion).

"Given the current situation, costs could run over 3 trillion yen ($29 billion)," the panel wrote in its report.

When it won the Games, Tokyo said they would bring in economic benefits of 3 trillion yen and create 150,000 jobs.

To cut back, the panel proposed reconsidering the construction of three new venues -- for volleyball, swimming and rowing/canoeing -- in favor of using existing venues.

SERIES OF BLUNDERS

For rowing and canoeing, they suggested three possible sites outside Tokyo, including a city in northeastern Miyagi prefecture, hours north of the capital.

Volleyball and swimming could be covered by renovating existing venues in downtown Tokyo, not far from the site of the Olympic Village, the panel said.

The rowing venue, estimated to cost 6.9 billion yen, now comes in at 49.1 billion yen, the panel noted.

But organizers expressed doubts, saying it might be difficult to win acceptance for changes.

"These sites were chosen over years and approved by the sports federations and International Olympic Committee," said Yoshiro Mori, head of the Tokyo 2020 organizing committee.

"It would be extremely difficult to overturn this."

Tokyo has already shifted venues for several events out of the capital, including sailing and basketball. Cycling will take place in Shizuoka, about 200 km (125 miles) west of Tokyo.

Tokyo organizers have been grappling with a series of blunders.

They were forced to scrap an initial design for the centerpiece National Stadium, site of the opening and closing ceremonies, because it was too expensive, and had to redesign the logo for the games following accusations of plagiarism.

($1 = 101.4000 yen)

(Editing by John O'Brien)