LONDON — First came the Brexit trade deal. Now comes the red tape and the institutional nitty gritty.

Four days after sealing a free trade agreement with the European Union, the British government warned businesses Monday to get ready for disruptions and “bumpy moments” when the new rules take effect on Thursday night.

Firms are scrambling to digest the details and implications of the 1,240-page deal sealed by the EU and the U.K. on Christmas Eve, just a week before the year-end deadline.

Ambassadors from the 27 EU nations, meanwhile, gave their unanimous approval to the deal on Monday.

“Green light,” said German spokesman Sebastian Fischer, whose country currently holds the EU presidency..

The approval had been expected, since all EU leaders have warmly welcomed the deal, which is designed to put post-Brexit relations between the bloc and former member Britain on reliable footing.

The agreement has not, however, eliminated the mistrust that festered between Britain and its neighbors during months of fractious negotiations

The French presidency said in a statement that France would remain “from the very first day very vigilant” about the implementation of the deal, especially to protect French companies and fisheries “in case the U.K. disregards its commitments.”

The agreement needs approval from Britain’s Parliament, which is scheduled to vote on it Wednesday, and from the EU’s legislature, which is not expected to take up the deal for weeks. The leaders of the European Parliament’s political groups said they would not seek full approval until March because of the specific and far-reaching implications of the agreement. The overwhelming expectation is that EU lawmakers will approve the deal.

The U.K. left the EU almost a year ago, but remained within the bloc’s economic embrace during a transition period that ends at midnight Brussels time — 11 p.m. in London — on Dec. 31.

The agreement, hammered out after more than nine months of tense negotiations, will ensure Britain and the 27-nation bloc can continue to trade in goods without tariffs or quotas. That should help protect the 660 billion pounds ($894 billion) in annual trade between the two sides, and the hundreds of thousands of jobs that rely on it.

But the end to Britain’s membership in the EU’s vast single market and customs union will still bring inconvenience and new expense for both individuals and businesses — from the need for tourists to have travel insurance to the millions of new customs declarations that firms will have to fill out.