The former president of a major Japanese advertising firm pleaded guilty Friday to bribing an Olympics official to secure a coveted sponsorship contract for the Tokyo Games.

The admission in a Tokyo court comes as the investigation into bid-rigging and bribery widens, casting a shadow over Sapporo's bid for the 2030 Winter Games.

"Nothing stated is wrong," Shinichi Ueno said in court at his first appearance, after prosecutors reeled off a list of allegations.

Prosecutors allege that Ueno, 69, offered at least 14 million yen ($103,000) to then-Tokyo Games executive Haruyuki Takahashi to secure sponsorship deals for ADK Holdings, Japan's third largest advertising agency.

By 2018, prosecutors said, Ueno was becoming "desperate" because his firm was yet to land a single contract, and pleaded with Takahashi: "please help us."

Takahashi is widely regarded as a pioneer of sports marketing in Japan, and his pet projects were prioritised by aides, prosecutors said.

He is now facing multiple charges in relation to a string of alleged bribes reportedly worth nearly 200 million yen.

After Ueno's indictment, ADK said in a statement that it takes the situation "very seriously" and vowed to "reconstruct our governance and compliance system".

The scandal has so far engulfed a suit retailer, a publishing firm and even a merchandise company licensed to sell soft toys of the cute Games mascots.

It has also prompted the northern city of Sapporo to suspend promotion of its 2030 Winter Olympics bid, and arrange a nationwide poll to gauge support.

Prosecutors are continuing to investigate the scandal, and just this month arrested another former senior Tokyo Olympics official and three businessmen.

The four men are accused of rigging a string of supposedly open competitive bids and limited tender contracts for Olympic events, reportedly worth a total of 40 billion yen ($305 million), local media said.

Questions had been raised over impropriety around the Tokyo Games even before the Covid-disrupted event took place in summer 2021.

The former head of Japan's Olympic Committee, Tsunekazu Takeda, stepped down in 2019 after French prosecutors launched an investigation into corruption allegations linked to Tokyo's Olympic bid.