Japan vowed on Monday to cooperate with a French probe into $2mn allegedly paid to help Tokyo secure the 2020 Olympics, as the son of ex-world athletics chief Lamine Diack denied receiving the money.

Prime Minister Shinzo Abe said he had ordered full cooperation with the French investigation into the payments, sent to a Singapore bank account which has been linked to Diack's son, Papa Massata Diack.

Japanese officials have been swift to deny wrongdoing in what is the most serious in a series of problems to affect the 2020 Games, including over the main stadium's design and the event's official logo.

French prosecutors said last week they suspect the payments were intended to help secure the 2020 Olympics for Tokyo, which beat out competition from Istanbul and Madrid.

Japan's Olympic chief last week insisted the payments were a "legitimate consultant's fee", while the top government spokesman insisted the bid was "clean".

And Prime Minister Abe told parliament on Monday: "I have instructed the education and sports minister to fully cooperate in the investigation."

"Education and sports minister Hiroshi Hase told the Japanese Olympic Committee and the former bid committee to cooperate in the investigation," he added, according to Jiji Press.

French prosecutors said some 2.8mn Singapore dollars ($2mn) were paid to the now defunct Black Tidings consulting company, which Britain's Guardian newspaper has linked to Papa Massata Diack.

Lamine Diack was an International Olympic Committee member in 2013 when Tokyo won the hosting rights for 2020. Diack and his son already face corruption charges in France.

'Let them investigate'  

The payments were discovered as part of an inquiry into allegations the Diacks organised bribes to cover up failed dope tests by Russian athletes, French prosecutors said. France became involved as the money may have been laundered in Paris.

But Diack's son, Papa Massata Diack, speaking to Kyodo News agency in his native Senegal, insisted he hadn't received any money from the Tokyo bid team.

"I haven't got any money," he said in Dakar. "Let them investigate... I have nothing to hide," he added.

He added that he had been friends with Ian Tan Tong Han, formerly the sole proprietor of Black Tidings, since the 2008 Beijing Olympics. But he said he didn't know Tan's company was contracted by the Japanese bid team.

"I've been in this sports business for 25 years. I know the rules," Diack said, adding that Tokyo's bid "shouldn't be tarnished" and had been done "very fairly".

Tsunekazu Takeda, the Japanese Olympic Committee president who led Tokyo's bid, said the money was for "professional services" for consulting work.

"I never knew there was a link (between the company and Papa Massata Diack)," Takeda told lawmakers in parliament on Monday. "Anyway, if it is in the realm of acquaintance there is no problem," he added.

"Internationally it is quite common" to have a contract with an international consultant, Takeda added.

The controversy comes after Tokyo had to scrap its original main stadium design due to its eye-watering price tag, and also had to weather plagiarism accusations over the Games' initial logo.