France has threatened retaliation if the UK does not respect Brexit commitments on fishing.
The deal requires an EU member state-flagged vessel to acquire a licence to fish in UK waters.
But so far only 22 of some 120 boats from Boulogne-sur-Mer, the first fishing port in the English Channel, have been given the go-ahead to fish in these waters.
Now Paris says it will block regulations that would allow UK financial firms to undertake business in the EU unless progress is made on fishing commitments.
"We are asking for the whole agreement, nothing but the agreement and as long as it is not implemented (...) we will take retaliatory measures in other sectors if necessary," Clément Beaune, France's European affairs minister said in an interview with BFM.
"The UK is expecting a number of authorisations from us on financial services. We will not give any until we have guarantees that, on fisheries and other issues, the UK respects its commitments," he added.
"It's give and take. It is necessary that each one respects its commitments, otherwise, we will be as brutal and difficult as necessary as partners."
Beaune added that if commitments aren’t met on fishing, “We will give none. It is quid pro quo.”
Fishing was one of the most controversial issues in Brexit negotiations as France led a fierce fight to allow continental fishermen to access UK waters.
The agreement on fishing has been slow to be implemented and is set to be a stumbling block until the end of negotiations on the Brexit deal, causing frustration among professionals and a call from France for firm European action.
In particular, it provides for the British to issue licences for the 6-12 nautical mile (11.11km - 22.22km) zone off their coast, where European fishermen traditionally went.
Northern French fishermen, who traditionally fished in British waters, have denounced the long delays in complying with the trade agreement when only 22 of some 120 boats from Boulogne-sur-Merl have been issued with such licences.
This situation has led France to devote a budget of €100 million in subsidies as part of a European aid plan for the fishing sector, one of those most adversely affected by Brexit.
MEPs will vote on the EU-UK trade deal on Wednesday which is expected to pass with an overwhelming majority.