French maritime police fined two British fishing vessels during a search operation on Wednesday.
A fresh row erupted between London and Paris over post-Brexit fishing rights.
French Navy Minister Annick Girardin tweeted that, although checks on British boats were standard during the scallop (oyster) season, they were also tightened "against the backdrop of hardening controls in the Canal, in the context of discussions on licensing with the US, UK and European Commission.
Girardin added that one of the British boats was caught fishing in Seine Bay without proper licenses.
However, the French Maritime Police escorted this boat and its crew to the port of Le Havre, where the harvest of fishing could be confiscated and the boat impounded. Penal sanctions can also be imposed on boat owners.
Earlier on Wednesday, France published a list of sanctions it could impose on the United Kingdom (UK) from November 2, unless progress is made in its fishing row with Britain after its exit from the European Union( EU).
On the other hand, the French ministries of Maritime and European Affairs said in a joint statement that "France can intensify its border and health checks on goods coming from Britain in particular.
The French authorities threaten to prevent British fishing boats from entering certain French ports and tighten inspections on trucks heading to and leaving the United Kingdom.
"The second round of measures is being prepared. France does not rule out a review of its electricity supply to the United Kingdom”, the joint statement issued by the two ministries added.
The UK has said threats to block British boats from French ports in a dispute over fishing rights are a breach of international law and trade agreements, according to BBC.
France said if there is no agreement by 2 November it will also tighten checks on UK boats and trucks, and could target Channel Island energy supplies. UK Brexit Minister Lord Frost said the stance was "disappointing". He said the UK was seeking "urgent clarification" of France's plans.