The Department for Transport (DfT) "still has much to do" as it prepares contingency measures for a no deal Brexit, Whitehall's spending watchdog has warned.
Further work is required to develop schemes such as issuing international driving permits and managing traffic flow at Dover, the National Audit Office (NAO) found.
Government advice is for all departments to have fully planned contingencies in place in the event of the UK withdrawing from the European Union in March 2019 without an agreement.
The NAO concluded that the DfT has made a "determined effort" to address the "significant and complex challenge" of Brexit.
But it warned that the department "still has much to do" and there is an increasing risk that projects will not be delivered on time.
Amyas Morse, the head of the NAO, said: "These are extraordinary times for the civil service and Government.
"The department has achieved a great deal in its preparations but over the coming months it will, like many other departments, need to scramble to prepare for the UK's EU exit, particularly if we are faced with a no deal scenario."
The NAO noted that plans are still being finalised for Project Brock, a Highways England scheme to deal with lorry queues on roads to Dover for cross-Channel journeys.
This will need to be ready by March when the UK leaves the EU, the watchdog said. The DfT told the NAO it is confident the project will be delivered on time.
The NAO said there is no detailed plan or business case for issuing international driving permits to UK motorists.
These may be needed to drive legally in the EU if no agreement is reached on mutual recognition of existing driving licences.
The DfT believes up to seven million permits could be requested in the first 12 months after Brexit.
The NAO also found that the DfT and the Department for Exiting the EU are measuring progress differently, with the DfT making a more cautious assessment on its work.
A Government spokesman said: "The NAO concludes that the department is making a determined effort to ensure the UK transport system is fully prepared for EU exit and acknowledges that the department has already achieved a great deal.
"We have prioritised preparation for EU exit and royal assent of the Haulage Permits and Trailer Registration Bill will be a significant step in this process."
The Bill deals with a post-Brexit lorry licensing system in the event of a no deal Brexit.
The spokesman added: "Our work is part of wider Government preparations to ensure the UK can deliver a smooth and orderly Brexit, as we move from our current membership of the EU to our future partnership."