LONDON — The British government issued its first contingency plans on Thursday for leaving the European Union without an agreement, seeking to prepare the public for possible disruptions without spreading alarm that could undermine support for the entire undertaking.

The government emphasized that it hoped and expected to hammer out a deal with the European Union. But in a series of technical documents, it warned that, without an agreement, importers and exporters could face significant new bureaucratic hurdles, that credit or debit card payments in Continental Europe could cost more, and that British citizens living in the bloc could lose access to banking and pension services.

Even minutiae like the design of cigarette packs would be affected, the government said.

Dominic Raab, secretary of state for Brexit, the term for Britain’s withdrawal from the European Union, is treading a diplomatic and political tightrope in issuing the warnings. To strengthen Britain’s negotiating hand, he wants to show that it is ready to walk away from crucial Brexit talks with the European Union this year if necessary.

But in trying to prove to his European interlocutors that Britain is prepared for all eventualities, Mr. Raab knows he risks frightening the British public and weakening faith in the Brexit project, which he supports.