TALLINN (Reuters) - The 27 countries that will remain in the European Union after Britain leaves must listen to each other carefully and avoid rushing into policy decisions, German Chancellor Angela Merkel said on Wednesday.

"This is a phase of listening, of understanding and learning from one another so that we can really understand and develop a new balance within the European Union of 27 members that will remain," Merkel, speaking at a joint news conference in Tallin with Estonian Prime Minister Taavi Roivas.

"If you do it wrong from the beginning and you don't listen, – and act just for the sake of acting - then you can make many mistakes," the conservative German leader said, underscoring that the upcoming EU summit in Bratislava was about setting an agenda for the future, not making any firm decisions.

She said it was important for larger and more economically powerful countries like Germany to understand how smaller EU members viewed the situation after Britain's June 23 vote to leave the bloc. "We have to consider what are our priorities are going to be and where we want to continue our efforts."Merkel reiterated that Britain must invoke Article 50, triggering the procedure for exiting the EU, before the remaining EU members can respond with their ideas on how to shape the future relationship between Britain and the EU.

The German chancellor said she remained calm about the likely timetable for Britain to take that next step, noting that remaining EU members had enough to do at the moment thinking about their post-Brexit future.

A UK government spokesman last week said British Prime Minister Theresa May will not begin formal divorce talks before the end of the year.

"We have enough to do among the 27 (remaining countries) looking at the questions about the future, so that we can afford to let Britain take the time it wants to clarify how it wants to shape the future relationship," Merkel said.

Merkel is meeting 15 other European heads of state this week to prepare the groundwork for a Sept. 16 summit in Bratislava aimed at shoring up the battered bloc.

The German leader also said she saw no fundamental obstacles to setting up joint military units with Estonia, much as Germany has already done with other countries, including France.

But the disparate sizes of the two militaries meant such an endeavor might require the participation of other countries besides Estonia, she said.

(Reporting by Andrea Shalal and Andreas Rinke; Editing by Caroline Copley/Richard Balmforth)