Hundreds of people heading for the Port of Dover have been forced to spend the night in their vehicles as police warned the severe delays could last until Monday evening.
Holidaymakers using the route to cross into France have been stranded for up to 15 hours in tailbacks since Friday evening after French authorities increased border checks.
The current delay is up to 10 hours, Kent County Council said.
This includes around five-and-a-half hours on the main eastbound road and a further wait in a buffer zone before reaching French border checks, which have been tightened following recent terror attacks.
Kent Police said the disruption is expected to continue until at least Monday morning and possibly into the evening.
Questions have been raised about staffing levels to deal with the huge increase in people travelling at the start of the summer getaway.
Port authorities said French border control booths at Dover had been "seriously understaffed", claiming coaches were at one point having to wait 40 minutes each for all passengers to be checked in as only one French officer was available.
A spokesman said concerns about staffing levels were raised with the Government earlier in the week and these had been passed to French authorities.
'Poor transport management'
The Home Office said as well as sending in the UK Border Force to help French authorities, Kent police will be "proactively managing" traffic to get drivers through more quickly.
A government spokeswoman said: "We recognise the security pressures that French law enforcement organisations are under at Dover and we have agreed the UK Border Force will assist the PAF (French border police) with border checks to remove the backlog.
"We understand that there has been extraordinary disruption in the Dover area ... but safety is paramount.
"Measures are also being taken on the approach to the port where Kent Police will be proactively managing traffic to speed up the process."
The Conservative MP for Dover, Charlie Elphicke, who was struck in traffic for around two hours on Friday evening, said there had been a lack of planning which led to "poor transport management".
He said the situation was "completely unacceptable" and should have been predicted, adding the people who are stuck should be given an apology.
