For weary Brexit negotiators on both sides of the English Channel, a Christmas Eve trade agreement sealed 11 months of painstaking deliberations over Britain¡¯s departure from the European Union, encompassing details as arcane as what species of fish could be caught by each side¡¯s boats in British waters.

But for many others ¡ª among them bankers, traders, truckers, architects and millions of migrants ¡ª Christmas was only the beginning, Day 1 of a high-stakes and unpredictable experiment in how to unstitch a tight web of commercial relations across Europe.

The deal, far from closing the book on Britain¡¯s tumultuous partnership with Europe, has opened a new one, beginning on its first pages with what analysts say will be the biggest overnight change in modern commercial relations.