A lorry park customs area being built to accommodate Brexit road congestion will not be completed for when it is needed on 1 January.
The Department for Transport confirmed on Tuesday that the customs control area at the site in Severington near Ashford in Kent will not be completed in time for the end of the transition period.
The DfT says an unplanned level of rainfall in England during the winter is responsible for work falling behind schedule.
Queues of up to 7,000 lorries are expected in Kent, home of the UK's channel ports, under worst-case scenario planning for changes to customs procedures at the end of the year.
The government's policy of leaving the single market and customs union will see the reintroduction of customs and other regulatory checks on trade with the EU, which had been eliminated by membership of the bloc.
Disruption is expected to be more acute if no deal is secured in time for the UK's economic Brexit, but the National Audit Office and trade experts have warned that there will be some no matter what the outcome.
The site at Severington will be available for the parking of lorries, but hauliers will have to initially move between two different sites to report for customs checks, causing additional disruption.
The revelation that the facility is still not finished is a blow for Boris Johnson, who claimed in August 2019 that the UK would be ready for no deal on 31 October 2019.
Ultimately, the prime minister's claim was never tested as he secured a withdrawal agreement which delayed much of the disruption to trade until the end of this year.
A spokesman for the DfT said: "From 1 January customs checks on HGVs will be taking place at the Ashford Waterbrook site before permanently moving to the Severington site in February 2021 if not before.
"The Severington site will open as planned on 1 January to manage traffic should there be disruption."
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