Bringing cigarettes or tobacco back to the UK from any other European country.
A new French law which came into effect on 1 January 2006, has ruled that the maximum amount of cigarettes that can be brought from any European country into France is 5 cartons or 1 Kg of tobacco. Cigarettes or tobacco exceeding this amount must be declared to the Customs authorities when entering France. If not a fine could be imposed as you will be considered as a commercial importer.
Info from D. Raddon.