The discussions between France and Germany have become so intense — with Germany wanting the deal done by Friday and France wanting to prolong the debate into Sunday — that there are some who worry that the Sunday meeting may not happen at all.

Spain, with its prime minister, Pedro Sánchez, ruling in a minority government, has responded to domestic criticism by insisting that it will oppose a Brexit deal unless there are guarantees that the future of Gibraltar, a British colony claimed by Spain, is left out of the agreement. His concern is that the colony’s future remains a bilateral issue.

Spain is likely to get some sort of guarantee or side-letter, known as a “declaration,” clarifying the point, and Spanish officials said on Wednesday that there had been some progress in easing their concerns.

Fish are also a sensitive issue. Future fishing quotas are so complicated that they were left out of the withdrawal agreement, postponed to the ensuing post-Brexit negotiations on the details of a future relationship. But the issue is important to all countries concerned, with even the British fishing industry divided between those who live on exports to the European Union and those who sell domestically.

Both Britons and Europeans are upset that if a separate deal on fish is not done by July 2020, during the transition period the European fishing industry will lose access to British waters while Britain’s will lose access to European markets.

There are other concerns, noted Charles Grant, the director of the Center for European Reform, a research institution. Some countries known for their pragmatism, like the Dutch, the Swedes and the Baltic countries, are working on legislation to cover important trading issues with Britain in the case of a no-deal Brexit. But the European Commission wants them to refrain from doing so now, to keep pressure on Britain to complete and ratify the withdrawal agreement rather than “work to help the British mitigate with mini-deals in advance of a possible no-deal,” Mr. Grant said.