Its latest report warns queues could be “much longer” in summer.
Much of the report is concerned with freight logistics, and the potential impact of thousands of trucks in Kent waiting to cross the Channel. Last month, the government confirmed: “Hauliers will need a Kent Access Permit to proceed to the border.”
But the NAO also highlights the impact of Brexit on private motorists.
The government’s reasonable worst-case scenario assumptions suggest that passenger queues could last up to two hours at Dover docks or Eurotunnel in January.
The report warns: “With an increase in the number of passengers travelling in the summer months, the queues and delays could become much longer.”
Edmund King, president of the AA, said: “We believe that further government guidance on Brexit travel issues is due in ‘the coming weeks’.
“Currently we still don’t know if drivers will need International Driving Permits or Green Cards.
‘Whilst potential delays for tourists, if allowed to travel, will be an inconvenience, it could be an absolute nightmare and potential commercial catastrophe for hauliers.”
The report adds: “Regardless of the outcome of the negotiations there will be significant change at the border from 31 December.
“Freedom of movement will end for people travelling between the UK and the EU. Passports with at least six months validity will be required for travel to the EU and all passports will be checked crossing the UK−EU border.
“Passengers might not be prepared for changes to controls, for example if forms of ID are no longer accepted, EU pet passports are no longer valid, or people can no longer pass through e-gates.”
Denmark removed from travel corridors list
The home secretary, Priti Patel, said: “When the transition period ends, we will have control of our borders and will deliver our new firmer and fairer points based immigration system.
“Our firm and fair approach will treat people from every part of the world equally.
“Phasing out the use of identity cards at the border, which are some of the least secure documents, is an important step in making our border safer.”
The Cabinet Office minister, Michael Gove, said: “Our updated Border Operating Model provides further detail on how the GB-EU border will work after the transition period ends on 31 December and the actions that traders, hauliers and passengers must take.
“Businesses need to prepare now for new procedures whether or not we reach a trade agreement with the EU, so that they can seize the significant opportunities that lie ahead.”
The NAO report concludes: “Despite the funding being committed by government, there remains significant uncertainty about whether preparations will be complete in time, and the impact if they are not.
“Some of this uncertainty could have been avoided, and better preparations made.”