Heightened French security checks have left ferry passengers at Dover waiting for more than four hours to get through passport controls.
Motorists in Kent have also been affected, with delays of up to eight and a half hours expected on the roads leading to Dover.
Backlogs have been most severe on the A20 and A2, though there have been reports that measures have been taken to split local, tourist and freight traffic.
"The disruption has been caused by a vast volume of holiday traffic and delays caused by increased security at the French border," Kent police said in a statement on Twitter.
The latest estimated travel times from P&O Ferries were 300 minutes for motorists travelling on the A20 and 180 minutes for those travelling on the A2.
DOVER UPDATE: estimated waiting times to get to the port:— P&O Ferries Updates (@POferriesupdate) July 23, 2016
A20: 300 minutes
A2: 180 minutes
once you get to the Buffer Zone: 30 minutes
In another tweet, the police service said it would deliver more than 11,000 bottles of water to drivers stuck in traffic.
Eurotunnel services have also been affected by "French government requirements to deliver heightened security checks", Highways England said in a statement.
Eurotunnel said this morning it was taking up to 90 minutes for passengers to check in.
The enhanced security procedure follows the attack in Nice last Thursday, in which a lorry was driven into crowds celebrating Bastille Day, killing 84 and injuring more than 100.
In a statement, the French Border Control at the Port of Dover said:
French Border Authorities have been operating at a heightened level of security.
However, the French border control booths have been seriously understaffed overnight with only three booths available for tourists out of a potential seven.
At one stage, only one French officer was available to check passengers on hundreds of coaches, resulting in each coach taking 40 minutes to process.
The Port of Dover, which has no authority over French border operations, raised concerns over French manning levels with the UK government earlier this week and the government, in turn, raised the issue with its French counterparts. The current wait remains lengthy and passengers should contact ferry operators for information, consider delaying their departure, and ensure they have plenty of water and food with them if deciding to travel.