UK trade with the EU faces "significant disruption" when the Brexit transition period ends in January, a government spending watchdog has said.
The National Audit Office (NAO) said it was "very unlikely" traders would be ready for checks the EU is due to impose at its borders.
It also warned "limited" time remained for UK ports to test new IT systems.
The government said "significant" efforts had been made to avoid disruption to businesses.
The UK is largely due to stop following EU trading rules from January 2021, having done so since formally leaving the bloc earlier this year.
Preparations are under way to prepare the border for the change, amid warnings the new systems might not be ready in time to avoid disruption.
In a report, the NAO said the end of the transition period would bring "significant change," even if the UK agrees a trade deal with the EU.
Even with a deal, traders will face new hurdles to clear - including the need to fill in customs declarations on goods being traded.
HM Revenue and Customs (HMRC) estimates it may need to process 270 million customs declarations from 2021, compared with 55 million currently.
In June, the government announced that - regardless of whether it reaches a post-Brexit trade deal - new checks on EU goods entering Great Britain would be phased in over six months from January 2021 to give firms "time to adjust".
However, the NAO said there was still "uncertainty" over where new border infrastructure would be located and whether it would be ready.
It also expressed concern traders would not be ready for the full checks on UK exports the EU is planning to implement from the start of 2021.
It cited the government's latest "worse case scenario" planning figures, which estimate 40% to 70% of lorries crossing the English Channel will not be ready.
In response to the NAO's report, a UK government spokesperson said it had invested £705m to ensure the "right border infrastructure, staffing and technology is in place".
"With fewer than two months to go, it's vital that businesses and citizens prepare too," they added.
"That's why we're intensifying our engagement with businesses and running a major public information campaign so they know exactly what they need to do to grasp the new opportunities available as the transition period ends."