Preparedness in Britain for a no-deal Brexit remains "at a low level", with logjams at Channel ports threatening to impact drug and food supplies, according to government assessments released Wednesday.
MPs voted last week to force the government to publish the no-deal "Operation Yellowhammer" document, which also warns of "public disorder" in such a scenario.
Britain's plan for no checks at the Irish border would likely "prove unsustainable due to significant economic, legal and biosecurity risks", it said, adding that it could lead to a black market developing in border communities, with dissident groups expected to capitalise.
The government stressed that it was "updating the assumptions" in the document, and that it was "neither an impact assessment, nor a prediction of what is most likely to happen.
"It describes what could occur in a reasonable worst case scenario," wrote minister Michael Gove.
But the warnings increase the pressure on Boris Johnson's embattled government, which has vowed to leave the EU on October 31, with or without a deal.
A Scottish court delivered him a blow earlier in the day, ruling that his controversial decision to suspend parliament in the run-up to Brexit was unlawful.
MPs also demanded to see private communications between government staff over the move to suspend parliament, but the request was rejected.
"The motion appears to direct the government to carry out searches that could only be discharged by breaching the legal framework set by parliament itself," wrote Gove.
The government said the request contravened the Investigatory Powers Act, the European Convention on Human rights and the Data Protection Act.
The Yellowhammer documents, which date from August 2, warned that up to 85 percent of British lorries may not be ready for French customs checks in the event of a no deal, reducing the "flow rate to 40-60 percent of current levels."
The worst disruption could last for three months, it added.
If unmitigated, "this will have an impact on the supply of medicines and medical supplies.
"Whilst some products can be stockpiled, others cannot due to short shelf lives."
It warned that "public and business readiness ...will remain at a low level... because of the absence of a clear decision on the form of EU exit."
Gibraltar could be particularly badly affected, it said, due to the imposition of checks at its border with Spain.
Disputes could also flare up in fishing waters, with non-British vessels still active in British waters, while "there may also be a rise in public disorder and community tensions," said the assessment.