In 2016, Consob said Hitachi had to raise its offer to 9.899 euros (8.58 pounds) per share from 9.5 euros when it launched a mandatory public offer to buy out Ansaldo STS minority shareholders. Hitachi had to launch the offer after first buying a 40 percent stake in the rail signalling company from state-controlled defence group Finmeccanica, now Leonardo.

The watchdog had said back in 2016 that Hitachi and the former Finmeccanica had colluded to keep the price of the offer artificially low by over-valuing another unlisted asset involved in the deal.

Consob's decision showed that both companies "were aware and wanted" to implement the deal in a way that was not in line with the "equal-treatment principle" that should be guaranteed to all shareholders, the ruling of the administrative court for the Lazio region showed.

Leonardo and Hitachi could not immediately be reached for comment.

(Reporting by Alberto Sisto, writing by Giulio Piovaccari; editing by Agnieszka Flak)