During the transition phase, the UK will no longer participate in EU decisions but will still be part of the single market and customs union until December 2020.
Britain and the European Union on Monday reached a landmark deal on a transition phase that will last for nearly two years after the historic Brexit divorce next year, EU negotiator Michel Barnier said.
The two sides also agreed on a "backstop" solution for the thorny issue of the future of the Irish border, just weeks after Prime Minister Theresa May said no British premier could ever accept such an idea.
After an "intensive" weekend of talks in Brussels, Barnier said the two sides were "in agreement on a large part" of the divorce treaty governing the terms of Britain's departure after four decades of membership.
"We have reached an agreement on the transition period," Barnier told a press conference with Davis.
The transition will begin on Brexit day – March 29, 2019 – and last until December 31, 2020, and during that period, Britain "will no longer participate in the European Union decision-making process," Barnier said.
"Nevertheless, it will preserve the benefits, the advantages of the single market and the customs union ... and will therefore be required to respect all the European rules just like all member states do."
Davis told the news conference that the transition deal "gives the certainty demanded by businesses and citizens across Britain and the European Union."
Both sides wanted the transition deal in place so that it could be signed off by EU leaders at a Brussels summit later this week, allowing the unlocking of talks on the future British-EU relationship in April.
The purpose of the transition period is twofold: to give businesses and citizens time to adjust to life after Brexit, and to give Britain and the EU more time to agree on a trade deal.
Britain also agreed in principle to the EU's "backstop" plan for the status of the Irish border, under which British-ruled Northern Ireland would remain part of the EU's customs union if there is no better idea.
The thorny issue will, however, require further talks to be fully resolved, the two sides said.
The EU insists that any divorce deal must ensure there is no "hard border" between EU member Ireland and Northern Ireland, saying it could compromise the 1998 peace accord that ended decades of sectarian violence in the British province.
Several weeks ago, May said "no UK prime minister could ever agree" to the plan, saying it would compromise Britain's constitutional integrity and effectively draw up a border between Northern Ireland and the rest of the United Kingdom.
The idea was also deemed "unacceptable" by the pro-British party in Northern Ireland that props up her government.
"We agreed today that the backstop solution must form part of the legal text of the withdrawal agreement," Barnier said.
The remaining 27 EU countries say Britain must agree to terms on Northern Ireland before it will sign off on the final version of their divorce agreement and advance to trade talks in April.
Irish Foreign Minister Simon Coveney, who met Barnier in Brussels, said on Twitter on Monday, "Brexit negotiations are moving forward."