High-level political agreement that EU and UK citizens must not be used as “bargaining chips” – a position David Davis, the Brexit Secretary said was a “moral responsibility” – would not be enough, EU sources said.

“It is not just a question of everyone agreeing ‘we’ll fix it’ then move on,” the senior EU source said.

“Political agreement is not enough. The details will have to be sorted before the EU side can move on.”

Chief among EU concerns is the question of the long-term “enforceability” of highly complex agreements that will cover pensions, the rights of spouses from third countries and other liabilities that could stretch 40 or 50 years into the future.

That will also bring up the question of the jurisdiction of the European Court of Justice, and whether Britain will consent to be bound by future ECJ rulings on disputes over the acquired rights of EU citizens that might arise in the future.

Mr Davis has denied claims that the UK is trying to "blackmail" the EU by threatening co-operation on security.

Guy Verhofstadt, the European Parliament's chief negotiator, on Thursday suggested that the Government was trying to use terrorism co-operation as a "bargaining" chip.

Mr Davis denied that the Government is resorting to "blackmail": "What the prime minister was saying was, if we have no deal - remember, what we want is a deal - it is bad for both of us."