DFDS said the freight-only service, which is the only ferry link between Scotland and mainland Europe, would operate with three departures per week from 2 May, rather than four.
But it added two ships would continue to operate the route.
Allan Hull, Route Director, said the change in frequency reflected a lower than expected demand for the service since it was changed from freight and passenger, operated using the Scottish Viking, to freight only at the beginning of this year.
Hull said: “It is unfortunate that we are not able to maintain the capacity we had hoped to operate on the route.
“In the current climate of escalating fuel prices, this meant we had no choice but to reduce the frequency of sailings.
“The position now is broadly the same as that offered during the time the Scottish Viking operated on the route, so there has been no net reduction in capacity to what was previously offered – or indeed in our commitment to the service.”
The service departs Rosyth and Zeebrugge on Mondays, Wednesdays and Saturdays.
The ships deployed on the service are the Tor Finlandia and the Tor Cimbria. Both vessels have capacity for around 130 trailers – the Scottish Viking could accommodate 120 trailers.