LONDON (AP) — Britain’s government will begin the process of leaving the European Union on March 29, starting the clock on the two years in which to complete the most important negotiation for a generation.

Britain’s ambassador to the 28-nation EU, Tim Barrow, informed the European Council President Donald Tusk of the timing on Monday morning, the Department for Exiting the European Union said.

The notification of triggering Article 50 of a key EU treaty will come in the form of a letter delivered to Tusk — though it was unclear whether it would come through an actual letter or an electronic missive.

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