French President Emmanuel Macron on Saturday said he was "not sure" it would be possible for the European Union and Britain to reach a trade deal by the end of the year.

Britain and the EU are about to embark on negotiations aiming to hammer out a trade agreement by the time the post-Brexit transition period shuts at the end of December.

But France has made clear it thinks the negotiations will be particularly difficult, especially in such a tight timeframe.

"I am not sure that an agreement will be reached between now and the end of the year," Macron said at a meeting with fishermen, who are concerned for their livelihoods after Brexit, at an agricultrual trade event in Paris.

"Anyway, it is going to become more tense because (the British) are very hard," he said, adding that fishing rights would be a key point of contention.

Britain formally ended its 47-year membership of the EU on January 31, nearly four years after a majority voted to leave the bloc in a 2016 referendum.

France and several other countries want to be able to keep fishing in British waters, while London wants full autonomy and limited access for European fishermen.

The EU's chief negotiator Michel Barnier has said the EU's top priorities are fishing, security and maintaining fair trading conditions for European companies.

He has also firmly rejected a British suggestion that City of London companies could be given broad, permanent access to EU markets without conditions.

Overall, French fishing boats generate 30 percent of their revenue from catches in British maritime territories, particularly rich in fish stocks.

French officials say that the UK exports the bulk of its catch to Europe, indicating that British fishermen have plenty to lose if the two sides fail to reach a deal.