BRUSSELS - British Prime Minister Theresa May welcomed on Friday the approval by European Union leaders of a 21-month transition period to help business adapt after Brexit, telling the bloc to ride the "new dynamic" in looming trade talks.

EU leaders also formally adopted on Friday their joint negotiating stance on a future trade relationship with Britain after it leaves the bloc, said the chairman of the summit, Donald Tusk.

"I welcome the fact that the EU Council this morning has agreed the details of the implementation period," May told reporters. "This gives certainty to people and businesses. It gives them the clarity to plan for their future."

"I believe there is a new dynamic now in the negotiations," she said. "We will now be sitting down and determining those workable solutions for Northern Ireland but also for our future security partnership and economic partnership."

The coup for May comes at the expense of having to kick into the long grass any fix to the politically sensitive issue of the Irish border after Brexit.

Both sides say they do not want to go back to border checks between the Republic of Ireland and Northern Ireland - as was the case during decades of violence in the British province.

Irish Prime Minister Leo Varadkar highlighted the EU stance that the transition would only become final as part of a broader deal between the bloc and London, which means they have to settle on all outstanding issues - including the Irish border - first.

"As Ireland ... we're not the ones who are leaving so we are not under time pressure in that regard," Varadkar told reporters. "Nothing is agreed until everything is agreed."

Under pressure from the EU and determined to get the interim transition deal to ease business concerns about the practical effects of Brexit, London agreed that its final agreement with the bloc would include an emergency backstop for Ireland.

For the EU that would mean continuing to treat Northern Ireland as if it remained inside the bloc's customs union even after the end of the transition period at the end of 2020. The EU says that would prevent border checks on the island.

May, who says Britain will also be leaving the EU's single market and the customs union, has strongly rejected that. But the two sides agreed some sort of emergency solution will be there to avoid an Irish border if everything else fails.

"If we can have an agreement on the terms backstop or an alternative to the backstop before June, that's something we would very much welcome," Varadkar said.

The Brexit schedule assumes the bloc and London would agree on the divorce deal, the transition and a framework for their future relationship in the summer so that the 27 EU leaders could endorse it at their summit in October and take it back to their national capitals for ratification, hopefully early next year and before the Brexit date.

The bloc's Brexit negotiator Michel Barnier said the new deal with Britain, talks on which are due to start next month, "will have to respect the principles and the identity of the EU and our single market".

Reuters