Countries like Australia have asked for trade compensation from the UK and EU over the interruption of Brexit.
Fifteen countries, including the US, India and New Zealand, raised concerns about Brexit at a World Trade Organization (WTO) meeting in Geneva.
Australian officials said their meat and lamb exporters had already been hit after several delays at Brexit.
Brazil said Brexit's plans for Northern Ireland could violate WTO rules.
The key issue for the fifteen countries is a system that allows them easier access to the EU's large market for limited quantities of some of their goods, especially agricultural products.
It is a system known as "tariff quotas".
Members of the World Trade Organization often apply tariffs – import taxes – to many of their products from abroad.
For some products, they are committed to allowing specified values to be imported at lower tariffs than normally applied. In some cases, the reduced fare is zero.
This makes it more profitable for Australian farmers to sell meat to Europe, for example.
Brexit complicates that.
Current quotas are for the whole EU, including the UK.
Brexit means that the United Kingdom and the EU need to decide how to divide them.
Some countries say this could lead to them having less favorable access than they currently have to what is a large and rich market.
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Australia said the UK and the EU have proposed different ways of allocating quotas that would result in less access than Australian exporters today.
The US argued that it could not have access to reduced tariffs for UK pizza cheese or grape juice for the EU.
Another concern is that the UK and the EU may end up using part or all of the other quota.
The US argued that this could severely affect its pork and wine sales.
Some countries are demanding compensation from the UK and the EU. In the WTO, this usually means reducing tariffs on other goods.
Australia has also argued that it has already been affected by Brexit.
A document prepared for the meeting said that when Brexit was still scheduled for October 31, many Australian companies stopped exporting valuable beef and sheep meat before Christmas because of uncertainty over whether to use these quotas.
The document said similar decisions were made before possible earlier Brexit dates and will have to be made again before January 31.
Brazil has raised another concern about the customs procedure plans between Northern Ireland and Great Britain.
Brazilian officials said this could violate WTO rules against discrimination.
The documents the BBC has seen do not explain why, but are probably based on plans for an open border between Northern Ireland in the United Kingdom and the Republic of Ireland in the EU.
Combined with the limited goods verification plans between Northern Ireland and the rest of the UK, it can provide an opportunity for goods to travel between the UK and the EU without restrictions or tariffs possibly applicable to similar goods from the rest of the UK. world. .
Brazilian authorities said it is essential that the principle of non-discrimination in the WTO be preserved.
They argued that it was not clear that the proposed arrangements for Northern Ireland would guarantee that this would happen.