The Dutch competition watchdog is investigating claims of a bunkering price-fixing cartel at ports in the Netherlands and Belgium.
Following a tip-off from the police the watchdog has visited dozens of bunkering-related companies at the ports of Rotterdam, Antwerp and Amsterdam in recent weeks.
The same Dutch body has also investigated price fixing among tugboat operators in Rotterdam in recent years.
No names have yet emerged from the investigation, which is still in its initial stages.