French prosecutors have reportedly widened their investigation into corruption in athletics to incorporate the two Olympic races awarded to Rio 2016 and Tokyo 2020 in October 2009 and September 2013 respectively.

It was claimed by The Guardian last week that the French investigation is "at an early stage" but it is likely to examine the role of Lamine Diack, the former International Association of Athletics Federations (IAAF) President, who served for 14-years as an International Olympic Committee (IOC) member before his term expired in 2013.

The Senegalese stepped down as an honorary IOC member last year after allegations he was at the centre of a cabal of IAAF officials who accepted payments in return for the covering up of failed doping tests, and was later arrested in France.

The decision by French prosecutors to investigate how Tokyo beat Madrid and Istanbul in the race to host the 2020 Games is likely to cause further embarrassment for Japanese organisers.

It follows the scrapping of original  plans for the main stadium last July over its $2 billion (£1.4 billion/€1.8 billion) price tag, and claims of plagiarism that forced organisers to ditch the Olympic logo.

On Friday (March 4), it emerged that a place for the Olympic Cauldron was not included in the plans for the Japanese capital's new stadium amid reports that it could pose a fire risk due to the amount of wood which will be used at the showpiece venue.

Muto remains assured that Tokyo 2020 is firmly on track, however.

"Preparations are going smoothly," he told AFP in an interview.

"It’s important to overcome hurdles one by one."

IOC President Thomas Bach has claimed there is "so far no evidence" to support claims of possible bribery in the bidding processes for Rio 2016 or Tokyo 2020.

Senior IOC member Richard Pound had raised the possibility in the second part of his World Anti-Doping Agency (WADA) Independent Commission report, published in January that Diack may have used his position as IAAF President and IOC member to try to influence the campaign for the 2020 Olympics.

Discussions between unnamed Turkish officials and Khalil Diack, a son of Lamine, concerning the bidding process, were referred to in a footnote of the report.

It had been suggested that a sponsorship deal paid by Japan, but not Turkey, may have led to Diack and allies voting for Tokyo rather than Istanbul.

"Tokyo 2020 considers that the allegation is beyond our understanding," said Muto.  

"We understand that the Games were awarded to Tokyo because the city presented the best bid."

Since being selected as hosts by the IOC in 2013, Tokyo has reportedly struggled to contain escalating costs with fears the total budget could exceed $15 billion (£10.5 billion/€13.6 billion).

This is six times more than that of the original plan and three times more than that of Rio 2016.

The Japanese Government scrapped the original stadium plan conceived by Iraqi-British architect Zaha Hadid amid public outcry over its estimated cost, which had doubled to ¥252 billion (£1.6 billion/$2.2 billion/€2 billion).

The new design will cost ¥149 billion (£920 million/$1.3 billion/€1.2 billion) and Muto has promised it will be finished in time for the Olympics, despite falling too far behind schedule to be used for the final of the 2019 Rugby World Cup.

"Of course the budget will get more expensive as a result of the rise in the cost of living and as plans change," added Muto, the former Deputy Governor of the Bank of Japan.

"It is unclear at this stage by how much.

"We will not get a complete picture of the overall figures until the Olympics are close.

"Construction costs in Japan are among the highest in the world.

"Given that, I believe it is a reasonable budget.

"However, we will keep a close eye on costs and make annual revisions to keep them under control.

"The public should bear as little burden for the overall cost as possible."