UK holidaymakers are delaying booking Channel crossings for next summer amid concerns about the consequences of Brexit, Brittany Ferries has said.
Forward bookings were down between 4% and 5% from some of its regular customers, the company said.
Nigel Wonnacott from Brittany Ferries said people were worried about the impact on areas such as pet travel, health insurance and driving licences.
The decline was "raising the red flag", he told BBC 5 live's Wake Up to Money.
Falls in forward bookings was affecting Brittany Ferries' cashflow, Mr Wonnacott said, and raised concerns about whether it would be able to fill its 10 ferries.
It will take delivery next year of a new vessel for the Portsmouth to Caen route, which accounts for about 30% of its traffic, that has capacity for up to 1,700 passengers.
Christophe Mathieu, the company's chief executive, said: "Two more ships will follow as part of our €450m investment in the future. But we need to make sure we have passengers and freight to fill these new ships."
"We know that uncertainty and instability in the UK will have consequences on both sides of the channel," he said. "A poor [Brexit] deal, or perhaps no deal at all, could impact Brittany, Normandy and the Loire."
Research from Abta, the trade body for tour operators and travel agents, found that 43% of people are confused about how Brexit will affect travel arrangements, up from 36% last year, while almost half (48%) fear it will make it harder to travel to the continent.
"As we head towards the peak holiday booking period, there is still a lack of certainty about what will happen when we leave the EU ... we encourage both sides to take a pragmatic approach to getting a deal done," an Abta spokesperson said.
Despite the concerns about Brexit, package holiday bookings for European destinations next summer were 14% higher than at the same point last year, the association added.
A spokesperson for Eurotunnel, which operates Le Shuttle services between Folkestone and Calais, said "bookings are strong going forward".
Some holidaymakers have told the BBC that concerns over Brexit are affecting their plans.
Graeme Murphy said: "We normally have the following year's travel and accommodation booked by now as we are independent travellers, but are waiting to see what visas, travel permits etc we will need as we are afraid that we may not have the necessary documentation in time to travel. Seems that our annual holiday to Lindos, Rhodes is not going to happen in 2019."
Mervyn Hill said: "We're not booking a holiday until the situation becomes absolutely clear... we need specific insurance cover for existing health issues, but we suspect the risk of a no-deal Brexit will have a significant detrimental effect on insurance premiums."
Brittany Ferries was founded by Breton farmers in 1972. It operates services between the UK and Ireland to France and Spain, with Britons visiting France and northern Spain accounting for 85% of passengers.
The company reported a 2% rise in passengers to almost 1.1 million for the three months to September compared with the same period last summer.