London (CNN) The UK has reached a joint legal agreement with the European Union on the legal terms of a Brexit transition deal, a milestone development in the tortured negotiations to extricate Britain from the bloc.

Under the terms, the UK will be able to negotiate trade deals with other countries during the 21-month transition, which begins when Britain officially leaves the EU at the end of March 2019.

But in order to secure the deal -- which gives a degree of certainty for businesses -- the UK was forced to make concessions, including on the rights of EU citizens who arrive during the transition period. London also had to accept a "backstop" deal to avoid a "hard" Irish border that would keep Northern Ireland subject to EU law.

Speaking at a news conference in Brussels on Monday, the EU's chief negotiator Michel Barnier said agreement on the draft legal text for withdrawal was a "decisive step" towards Brexit.

He said the text, to be presented to EU heads of government on Friday, will be "the international agreement for an ordered withdrawal of the UK from the EU."

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