Dover could lose £1bn worth of trade a week if Britain leaves the EU without a deal, the port has said.

Doug Bannister, the chief executive at the port of Dover, told a fringe event at the Conservative conference that the government’s assumed drop in traffic under a no-deal scenario would cut £1bn a week from the flow of goods.

He said the port had made extensive preparations for 31 October to function as usual but added: “That’s how critical it is. If there’s a no-deal Brexit, it’s not going to be OK. But people are doing all they can to ensure Britain keeps trading.”

The transport minister, George Freeman, confirmed the government’s “planning assumption” was that no-deal disruption would roughly halve the traffic – a fall of 40%-60% – able to pass through Britain’s main trading link for three months.

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Dover handles £122bn a year by value, or 17% of the UK’s trade in goods. A fleet of 12 ferries carries more lorries than all other UK ports combined, on the shortest sea crossing between the UK and the EU.

Freeman said he hoped goodwill would prevail if a deal was not agreed by 31 October, averting worst case disruption. Asked what preparations the government was making for “malign interventions”, such as a potential blockade by French fishermen, Freeman said he had seen cabinet papers showing that “the ministry of defence are actively looking at it”.

The Freight Transport Association(FTA) said it did not expect “wholesale meltdown” but British consumers could expect to see less fresh produce. James Hookham, the deputy chief executive of the FTA, said: “There will be changes to what people can expect to see in the shops, undoubtedly.”

The FTA plan to hold 140 training sessions in the next few weeks to educate hauliers about the potential paperwork needed. Hookham said questions still remained unanswered, even with the intricate flowchart issued by the Department for Transport to explain customs and border processes.

“The biggest unknown in our book is what the French and EU will demand in terms of British goods imported into the EU,” he said.

Hookham added that the full impact of a no-deal Brexit on Dover, Calais and Kent would probably not be seen until 4 November, due to the public holiday on 1 November.