Carney told a news conference that surveys suggest that less than half of U.K. businesses have initiated contingencies plans for the possibility of a no-deal Brexit. He noted that 250,000 business owners have never completed a customs declaration.
Carney highlighted the need for Britain to secure a transition period during which it follows EU rules even though it has left the bloc. He said the transition should be "as long as necessary to prepare properly for the new trading relationships, but no longer."
Prime Minister Theresa May's Brexit deal, which is in danger of not being passed by Parliament, has a transition period through to the end of 2020 but the EU has said it can be extended.
The Bank of England says a smoother Brexit for Britain could help mitigate much of the economic impact from the country's decision in June 2016 to leave the European Union.
As well as publishing forecasts for a disorderly Brexit, the central bank played out would happen to the British economy in the event of a deal between Britain and the EU. It simulated the impact of an exit with no frictions between trade with the EU, as well as with such frictions.
The deal that British Prime Minister Theresa May has secured with the EU is unclear about the shape of the future trade relationship.
If Britain retains a "close economic partnership" with the EU, the bank said the British economy will only end up being around 1.25 percent smaller by the end of 2023 compared with the country's trend growth rate before the vote to leave the EU. A "close partnership" assumes no impediments to trade in goods and some trade in business and financial services.
The Bank of England has warned that the British economy could shrink by a massive 8 percent in a matter of months if the country leaves the EU without a deal next year, and that unemployment could nearly double.
In an analysis requested by British lawmakers, the central bank simulated a worst-case scenario involving Britain crashing out of the European Union in March 2019 with no deal on future relations like trade, and no transition period to help businesses adapt.
A recession on that scale would be Britain's deepest in modern history and far beyond the one experienced after the global financial crisis a decade ago.
The bank predicts inflation would rise to almost 7 percent next year as import prices rise following a collapse in the value of the pound. House prices are estimated to fall by 30 percent.
It said the financial sector could withstand the shock as banks have built up capital buffers.
The bank assumed a number of worst-case scenarios in its prediction, including the imposition of tariffs and other barriers to trade on all trade with the EU.
The European Commission is rejecting a petition demanding that all EU citizens be given a say on whether Britain can leave the bloc.
The petition's organizers want the Commission to allow everyone in the 28 member states to vote on Brexit in an EU-wide referendum.
The Commission, the executive body that supervises EU laws, said Wednesday that it cannot register the demand because the bloc's treaties permit countries to leave if they want.
Commission Vice-President Valdis Dombrovskis said: "This matter simply falls outside our field of competence."
Citizens' initiatives allow 1 million people from at least one quarter of EU member countries to request that the Commission take action where it has powers to do so.
The Commission says it regrets Britain's departure but must respect the 2016 Brexit referendum.
A British government analysis says the country will be poorer after leaving the European Union than if it had stayed in, no matter what sort of trade deal it has with the bloc.
An assessment released Wednesday estimates that 15 years from the day of departure, GDP will be 0.6 percent lower than it would have been if the country stayed in the EU — if the U.K. maintains frictionless trade with the bloc. If it leaves without a deal and there are significant barriers to trade, the economy would be up to 9.3 percent smaller.
The data analyses a range of options, from leaving without a deal to staying in the EU's single market for goods and services.
The analysis comes as Prime Minister Theresa May battles to persuade lawmakers to back the divorce deal the government has struck with the EU.
It does not analyze the specific deal agreed last week, but estimates that under similar terms the economic hit would be at the lower end of the range.
Britain's Treasury chief says leaving the European Union on any terms will leave the economy worse off than remaining in the bloc.
Philip Hammond says that "if you look at this purely from an economic point of view there will be a cost to leaving the European Union, because there will be impediments to our trade."
But he says a divorce deal agreed between the British government and the EU, which calls for continued close economic ties, will minimize the economic damage.
The government and the Bank of England both plan to publish assessments Wednesday of the economic impact of Brexit under different scenarios.
Prime Minister Theresa May is battling to persuade skeptical lawmakers to back the Brexit deal before Parliament votes Dec. 11 to accept or reject it.