(Decanter) - French wine broker and producer Castel has been hit with a €4m (US$5.46m) fine for not telling France's competition watchdog that it had bought Burgundy's Patriarche.

Castel withheld its acquisition of Patriarche in May 2011 from competition officials, which meant they were unable to check whether the deal placed too much wine market power in Castel's hands, France's competion authority said in a verdict delivered last week.

It has fined Castel's parent group, Copagef, €4m, to reflect the 'seriousness' of the case. It is believed to be only the third case of this nature that the watchdog has dealt with.

Officials said they only discovered Castel's acquisition of Patriarche in September 2011, after they were notified by a third party contractor tasked with investigating the French wine group's deal to gain control of another firm, Quartier Francais Spiriteux.

The competition authority subsequently investigated and approved the Patriarche deal in July 2012.