Dover and other Channel ports face disruption of up to six months if the UK leaves the EU without a deal, ministers have said.
The revised warning comes after technical analysis by officials on traffic flows if there are delays with customs checks in a no-deal Brexit.
While this is regarded as a worst case scenario, the public sector is being asked to rewrite its contingency plans.
NHS leaders are being urged to check plans medical supplies can be secured.
Source: The Guardian