BRUSSELS, Belgium — Here's how the process for agreeing on a delay to Britain's departure from the European Union works.

On Saturday, Boris Johnson wrote to EU Council President Donald Tusk asking for a three-month extension to the Brexit negotiations until January 31.

The EU’s approval isn’t automatic and requires the unanimous agreement of national leaders. Indeed, French President Emmanuel Macron has threatened to block it, as he did Britain’s last request for an extension.

Ambassadors from the EU’s other 27 governments will meet in Brussels at 5:30 p.m. local time on Wednesday to discuss the UK’s request. They may decide to hold further meetings before making their recommendation. The diplomats can’t make the final decision — that will have to come from Tusk on behalf of all the leaders — but they will give a sense of what the EU’s response will be.

It could be that the decision is a formality or, if there’s disagreement over how long the extension should be, Tusk will have to convene a summit. EU officials say they expect a final decision on Friday — either to grant the extension or call a summit.

By Ian Wishart and Peter Flanagan; editors: Edward Evans and Paul Sillitoe.