BRUSSELS (Reuters) - The 27 remaining nations of the European Union will start working on how they see their future ties with Britain next week, officials and diplomats said, as they pile pressure on London over the slow pace of Brexit negotiations.

An Anti-Brexit protestor waves EU and Union flags outside the Houses of Parliament in London, Britain December 5, 2017. REUTERS/Simon Dawson

The chairman of EU leaders, Donald Tusk, will meet British Prime Minister Theresa May in London on Thursday, a day after the bloc’s Brexit negotiator weakened sterling by issuing another warning to Britain, which is due to leave the bloc in March 2019.

May is due to make a policy speech on Friday to outline what sort of relationship London wants with the EU after Brexit and a subsequent transition period, which the bloc wants to see ending at the end of 2020.

But the EU, frustrated by weeks of prodding London to present its stance with little sign of success, has now decided to move on with its internal preparations on future ties.

It will only take into account what it has heard from Britain so far, namely that it will be leaving the bloc’s single market and customs union eventually as well.

“I expect Tusk to be ready with these draft guidelines on the future framework early next week,” said his spokesman, Preben Aamann.

EU states would then work on the document to have it approved by their 27 leaders when they next meet in Brussels on March 22-23.

The bloc has dismissed London’s proposals on the matter so far as “pure illusion” and said that they would not prevent a border between Ireland - an EU state - and Britain’s province of Northern Ireland, which both sides have vowed to avoid.

EU Brexit negotiator Michel Barnier on Wednesday presented a more advanced contingency plan for Ireland, drawing fire from Britain for proposing that Northern Ireland remain closely linked with the EU, which could loosen its ties with the UK.

“Let’s see what May says on Friday. If it’s another speech about what they don’t want, then this is not helpful. What we are looking for is constructive proposals,” a senior EU diplomat said.

“Since they’ve come up with nothing, we will present something. We will put forward a proposal of a free trade agreement. And they won’t like that either. So we will have a lot of things on the table.”

Brussels hopes the EU will be able to agree rules on the transition period with Britain in March and open negotiations on the future relationship from April. Negotiators for both sides are due to hold a week of talks in Brussels next week.