People travelling from the Port of Dover this week have been warned to expect two-and-a-half hour delays.
The Kent port issued the alert ahead of many schools in England and Wales breaking up for summer on Friday.
Enhanced post-Brexit passport checks by French border officials have significantly increased processing times.
Port bosses have taken a series of measures aimed at minimising queues.
The installation of a booth for French border officials Police Aux Frontieres (PAF) is expected to be completed before Friday and hour-by-hour traffic flow projections have been created.
The port is also continuing to use a new system for processing coach passengers, involving Advanced Passenger Information checks being carried out away from the main port facility during busy periods.
It was first introduced for the May half-term after some coach passengers were stuck at the port for more than 10 hours during Easter.
The port is urging car passengers embarking on summer getaways from Friday to arrive three hours before their booked departure – up from two hours last year.
Before the end of the Brexit transition period the port gave no advice for when people should turn up.
Border controls anticipated to take around 90 minutes
Port of Dover chief executive Doug Bannister said: “When passengers arrive at the Port of Dover, they will be processed through border controls – anticipated to take around 90 minutes on peak days.
“However, our modelling indicates that processing times may be up to two-and-a-half hours during the peak hours of 6am to 1pm during the first couple of Saturdays and Sundays of the summer holidays, due to the extreme popularity of these days.
“We care about every passenger that chooses to travel via the Port of Dover and have worked hard to ensure the best experience possible as they transit through on their summer holidays.”
Image: Port of Dover chief executive Doug Bannister ‘There will be some queueing’
Mr Bannister told the PA news agency his staff have done “everything we possibly can” to minimise delays.
He added: “People would anticipate if they’re travelling at very popular times on very busy days there will be some queueing.
“We can’t plan for unforeseen circumstances such as technical issues, industrial action or road traffic accidents on the approaches into the port.
“If these happen that could throw us some more challenges.
“What Dover has demonstrated time and time again is that we can be very resilient in our operations and clear the disruption very quickly.”