The Government has admitted it is preparing for the "reasonable worst case" scenario ahead of Brexit as a leaked report warned of queues of 7,000 lorries in Kent and significant delays to cross into the EU.
A confidential document prepared by the Border and Protocol Delivery Group, and seen by The Guardian, also predicts thousands of passengers could have to wait an additional two hours for Eurostar trains.
But a Cabinet Office spokeswoman said in a statement the Government was using a "stretching scenario" as opposed to a prediction.
She said: "As a responsible government we continue to make extensive preparations for a wide range of scenarios, including the reasonable worst case.
"This is not a forecast or prediction of what will happen but rather a stretching scenario. It reflects a responsible government ensuring we are ready for all eventualities."
The 46-page report, dated last week, says an essential IT system used by hauliers will not be tested publicly until the end of November, one month before the UK's transition phase with Brussels ends.
Leading union Unite warned on Monday that Britain's ports would be plunged into "chaos and confusion" in the new year unless customs systems and lorry parks are completed.
The union said lorry drivers feared the complex computer software to deal with customs would not be ready by December 31, adding most of the planned lorry parks were still to be built.
The latest developments come as Boris Johnson's controversial plan to override key elements of the Brexit deal he signed with Brussels cleared its first Commons hurdle despite deep misgivings by some senior Tories.
