Peter Wilding, who first came up with the "B" word and who is now Brexit director of law firm FBC Manby Bowdler, warns that "Brexit is not done".
"It hasn't even started," he said. "Number 10 might want us all to think that my word Brexit should now put in the file marked completed when the bongs of Big Ben didn't strike 11pm last night, but don’t be deceived.
"For anyone who wants to travel in or trade with the European Union things are going to get a lot harder."
The country has now entered an 11-month transitional period in which the relationship between Britain and the EU remains the same until the end of the year, by when a new deal must be ratified or Britain leaves the union without a deal.
But Mr Wilding warns people need to prepare for the fact that British citizens will lose several rights as EU citizens next year.
"You will need to renew your British passport now if you’re travelling to Europe next year because on the day you travel, you’ll need your passport to both have at least six months left and be less than 10 years old," he said.
"If you do not renew it, you will be turned back at passport control in all EU countries and Iceland, Liechtenstein, Norway and Switzerland."
Holidaymakers, he said, will also have to spend £6.30 on a three-year pre-travel visa called European Travel Information and Authorisation System before travelling.
Mr Wilding also warned that unless the UK re-enters the Single Euro Payments Area as part of the negotiations, the cost of card payments in EU countries will increase as they will no longer be covered by a ban on businesses charging customers for using their cards.
Consumers shopping in the EU or buying online from an EU company with a UK card could also be hit with surprise charges.
And from next year, European Health Insurance Cards will no longer be valid.
Mr Wilding said: "This means that if you need medical care abroad you will have to pay for it because currently most travel insurance policies won’t cover anything but emergency illnesses.
"If you don’t have health cover some countries will turn you back."
Driving in Europe will also change, with a £5.50 international driving permit required, along with a Green Card which takes more than a month to get from your vehicle insurance company, and a GB sticker.
Pet passports will also be affected, Mr Wilding said, meaning blood samples must be sent four months before travel to an EU-based vaccination unit, and taking your animal to a vet at most 10 days before travelling.
Border controls, he says, will change, as you'll need to show a return or onward ticket, show you have enough money for your stay and, travellers will no longer be fast-tracked at passport control.
Finally, using your mobile in the EU will cost more, as free roaming is under an agreement with the EU.
"These are the things you’re going to have to think about before you travel to Europe, not when you’re walking with your family through the departure lounge," Mr Wilding said.
"Which is why with Brexit, in the words of the Carpenters, we’ve only just begun."