Given the entry into force of the withdrawal agreement on 31 January 2020, travel requirements will remain the same as they are at present through to the end of the transition period: you can enter France with a valid passport. A visa is not necessary.
The travel requirements that will apply after the transition period will depend on the outcome of the future relationship negotiations underway.
During the transition period, travel requirements will remain the same as they are at present: travellers must be in possession of a valid passport or national identity card. Visas will not be required for either short stays (less than three months) or long stays.
Following the transition period and if no other agreement comes into effect, British citizens who travel to France in the post-transition period will be treated as non-EU citizens, and will be subject to more stringent controls at border crossings. British travellers will also need a long-stay visa if they wish to stay more than three months.
For shorter stays, British travellers will need a passport and will not need a visa, in accordance with a regulation be adopted at EU level provided that the UK reciprocally grants visa-free travel to all EU citizens.
British family members of British citizens already living in France prior to 31 December 2020 will be issued with a visa free of charge and will be entitled to a residence permit when they arrive in France in accordance with the withdrawal agreement. Other measures may be taken to streamline border crossings.