(Adds details from report in pars 4 and 5)
* Taisei asked rival not to bid for a project-Nikkei
* Four firms allegedly agreed to collude since around 2011-Yomiuri
TOKYO, Dec 20 (Reuters) - Shares of Taisei Corp fell more than 4 percent on Wednesday after a media report that the Japanese construction firm had asked rival Obayashi Corp to refrain from bidding on a project for magnetic levitation (maglev) trains.
Taisei had asked Obayashi not to bid for tunnel work in the $80 billion high-speed rail project, saying it and another competitor, Kajima Corp, planned to take the project, the Nikkei business daily said. The work was eventually awarded to Taisei and Kajima.
Tokyo prosecutors are investigating the three firms, as well as Shimizu Corp, for suspected antitrust breaches relating to the high-profile maglev project. The four companies have said they would cooperate fully with authorities, while declining to provide further details.
Yomiuri newspaper said company officials allegedly had agreed to divvy up contracts for the project since around 2011, when the maglev route was determined. The officials had known each other personally for years, the newspaper said.
Shares of Taisei and Kajima were down 4.4 percent and 2.5 percent, respectively, in early trade, underperforming a roughly flat Tokyo market. (Reporting by Taiga Uranaka; Editing by Chang-Ran Kim and Stephen Coates)