"October is only five months from now and still some key issues related to the withdrawal agreement need to be settled. In June we need to see substantive progress on Ireland, on governance and all remaining separation issues," said Bulgarian Deputy Prime Minister Ekaterina Zakharieva, which holds the EU's rotating presidency.

German, Austrian and Dutch ministers all echoed the same concern, saying Britain has not made its position clear in detail on parts of the negotiations.

At home, May is stuck between a rock and a hard place with staunch Brexit supporters pushing to sever ties with the EU and others advocating keeping close customs cooperation with the bloc to reduce frictions in future trade.

May's spokesman said London was working on two options for post-Brexit customs cooperation.

Under a customs partnership, Britain could collect tariffs on goods entering the country on the EU's behalf. Under a second idea, for a streamlined customs arrangement, traders on an approved list would be able to cross borders freely with the aid of automated technology.

But the EU has said London must come up with a solution for the Irish border conundrum and highlights that has not happened. Both sides worry that reinstating a physical border between EU-member Ireland and Britain's province of Northern Ireland - including to manage customs - could revive violence there.

Other outstanding issues include guarantees for expatriate rights, agreeing on security cooperation and trade rules after Brexit.

With May's cabinet, her ruling Conservative party and the British split on Brexit, the prime minister has come under increasing pressure at home in recent weeks to make a decision on customs.

Reuters