Automotive industry, agriculture, financial services. Several sectors of the European Union (EU) would be particularly affected in the event of a divorce between the United Kingdom and its 27 non-agreement partners.
A "non-agreement" would result in the introduction of barriers to trade with the reimposition of customs duties and the non-mutual recognition of environmental or sanitary standards, for example.
The automotive industry is the sector where damages could be more important. Professionals have warned on several occasions about the possible disastrous effects in an industry that employs almost 12.2 million people in the continent.
About 10% of the European automotive industry's exports go to the United Kingdom, recalls Vincent Vicard, an economist at the Center for Prospective and International Information Studies (CEPII).
In addition, the UK and EU automotive industries are highly integrated. "Sometimes some auto parts cross the border between the UK and the mainland five or six times," explains Carsten Brzeski of ING Diba. Any dysfunction in the supply chain, such as the passage of customs, would harm them.
A brutal divorce would particularly affect the German auto industry, which is well established in the UK. "If our supply chain stops at the border, we can not continue to produce in the UK," said Stephan Freismuth of the German BMW.
Meanwhile, the chemicals industry would also be affected. "Many German, French, Dutch or Belgian companies have production sites" in the United Kingdom, Brzeski says. Therefore, the production chain would also be particularly affected.
In addition, multinationals such as the Anglo-Dutch petrochemical group Shell or the Anglo-Dutch Dutch LyondellBasell also face specific problems related to their governance.
Agriculture and fishing would not be left behind either. Currently, only 60% of British food needs are covered by its own production, the rest is imported mainly from France, Belgium, the Netherlands and Ireland.
If customs duties are reintroduced, goods will be more expensive and their entry into the United Kingdom may be postponed. "We can imagine that there are trucks blocked in Calais and that, with the help of waiting, the milk transports the sour before arriving at Dover," suggests Brzeski.
Entry of many goods and animals may be prohibited unless it is registered in the United Kingdom in the list of authorized third countries. This registration can be made quickly, with conditions, for a former EU member.
Fishing could also be a problem for France, Spain, Portugal, Denmark or the Netherlands, whose fleets usually operate in British territorial waters.
The aviation industry would also be among those affected. Airbus, which manufactures its aircraft at different production sites throughout the EU, has already issued the alarm in case of a Brexit without an agreement.
The European group, which directly employs nearly 15,000 people in the United Kingdom, where it manufactures the wings of its handsets, has warned that an abrupt exit from the bloc will be "catastrophic" and will force the company to question its investments in the country.
In July, German Tom Enders, chief executive of Airbus, expressed concern about the departure of the UK from the European Aviation Safety Agency (EASA). "Beginning next April, certifications of thousands of parts of our aircraft would no longer be valid, which could mean a drop in our production," he said.
The fears of a sudden brake on air traffic between the UK and the mainland make it a particularly sensitive sector.
The European Commission wants to ensure that companies can fly over European territory and that safety certificates remain valid for a limited period. This needs an agreement in the UK.
On the other hand, financial services also look carefully at the events behind Brexit. Operators based in the United Kingdom will lose their right ("financial passport") to provide their services in the 27 countries of the bloc without being present.
The Commission has already noted that many operators have done what is necessary to "adjust their contracts and relocate" activities on the continent.
On Friday, the Governor of the Bank of France demanded to maintain vigilance in the compensation sector, since a divorce without agreement could "pose a risk" to the stability of the financial system.
British companies have almost a monopoly in this activity, which is to keep the accounting of exchanges between financial agents on world markets and ensure the smooth execution of transactions between all operators.