Citing a source "familiar with the matter", Japanese news agency Kyodo News reports that "several officials" at Dentsu made the admission but claimed that they did not know that it was an illegal act when the rigging began.

It is reported that the admissions came during voluntary questions by prosecutors.

Several staff at Dentsu are suspected of playing a role in the bid rigging with some seconded to work during Tokyo 2020, according to Kyodo.

The headquarters of Dentsu and fellow advertising agency ADK Holdings and Hakuhodo were raided in November following suspicions that 26 bids for the rights to organise 56 test events conducted between 2018 and 2021 were rigged.

The source told Kyodo News that Dentsu were tasked by Tokyo 2020 organisers with putting together a list of advertising agencies and other companies to categorise them into groups by respective track records in each sport.

It has previously been reported that the contracts for the events to prepare for the Olympics totalled JPY500 million (£3 million/$3.6 million/€3.5 million).

ADK, the third-largest company in its industry, won three bids totalling JPY104,000,000 (£646,000/$800,000/€737,000), while Dentsu - the largest advertising agency in Japan - won one for JPY11,000,000 (£66,000/$84,700/€77,000).

The first reports of corruption came when former Tokyo 2020 Organising Committee member Haruyuki Takahashi was alleged to have taken bribes to the tune of JPY200 million (£1.2 million/$1.4 million/€1.3 million) from five companies.