Canada's free trade deal with the European Union will form the basis for a swift transition to a post-Brexit trading relationship between Canada and the U.K., Prime Minister Justin Trudeau and his British counterpart Theresa May suggested Monday.

Trudeau and May spoke to reporters after a late-morning meeting on Parliament Hill.

The two leaders told reporters they agreed to a "seamless" transition of the Comprehensive Economic and Trade Agreement (CETA) between the European Union and Canada after the U.K. leaves the EU and is no longer a party to that agreement.

The U.K. is Canada's biggest export partner in Europe.

May said her government will be putting together a "working group" with Canada on a new trade deal, with CETA as the basis.

'The UK cannot start negotiating any trade agreement until 2019' after Brexit, says EU Trade Commissioner Cecilia Malmstrom. 6:32

"We believe it makes sense to take the trade agreements the U.K. is part of as part of the European Union with Canada and say that's the basis, at the point we leave, for a bilateral relationship between the U.K. and Canada," she said.

"Of course, over time, it would then be possible if we chose to change the details of that relationship to do so."

May said the U.K. has already discussed this with the EU.

PM Justin Trudeau and U.K. PM Theresa May discuss the transition required for future trade between Canada and the U.K. in a post-Brexit world. 1:18

Trudeau agreed there could be changes to shape this new deal and make it particular to the relationship between Canada and the U.K. after the initial post-Brexit transition.

"We will be able to move forward in a smooth transition that keeps the essence of CETA applicable to the U.K. in ways that respect the EU's requirements and rules," he said.

CETA, which was approved in February after years of talks, officially comes into effect on Thursday.

In late March the U.K. triggered a two-year window to negotiate Brexit, although there's a possibility that could be extended.