Failure to complete a new customs system by the date of Brexit in 2019 will cause massive queues at Dover, a report has warned.
The Public Accounts Committee warned it would be "catastrophic" if the system was not ready in time and there was no viable fall-back option in place.
It said huge disruption would follow for businesses, with food being left to rot in trucks at the border.
In 2015, around 55 million customs declarations were made by 141,000 traders.
That figure could increase five-fold to 255 million following the UK's exit from the EU, requiring the new system – the Customs Declaration Service (CDS) – to be in operation and Treasury funding secured for contingency options, the report states.
It says: "This is a programme of national importance that could have a huge reputational impact for the UK if it is not delivered successfully.
"The uncertainty regarding the outcome of UK-EU negotiations is a complicating factor but it should not be used by HMRC to avoid taking action now in areas including: scaling up the CDS to handle 255 million declarations; ensuring a viable contingency option is in place well before January 2019; and communicating with traders.
"There are financial as well as operational implications of not acting now. This is a tight timetable at the best of times. With the hard deadline of Brexit, delay is not an option."