British Prime Minister Theresa May was scheduled to address EU leaders in Brussels on Wednesday evening in what had been previously billed as a "moment of truth" for Brexit negotiations.

But expectations of a breakthrough are low, as there are growing concerns the two sides will be unable to bridge the gap over the issue of the Irish border.

May's official spokesman said she would take the opportunity to set out the areas where progress had been achieved while stressing her commitment to finding an agreement.

Yesterday was the date set for EU member state leaders to give the green light for a special summit to finalize the terms of Britain's withdrawal in November.

European Council President Donald Tusk warned that without new "concrete proposals" from the UK to break the gridlock over the Irish border "backstop" further progress may be impossible.

During a meeting with her cabinet on Tuesday, May insisted she would not accept an agreement on the backstop which undermined the integrity of the UK or tied it indefinitely to EU customs arrangements.

May told her colleagues: "If we as a government stand together and stand firm, we can achieve this."

A Downing Street statement said cabinet members endorsed May's call to "maintain the integrity of the union" between Great Britain and Northern Ireland, which the prime minister told the cabinet was threatened by the EU's proposed version of the backstop.

A spokesman said the prime minister had told her political colleagues it was "not possible for her or any UK prime minister to sign up to an arrangement that would lead to a customs border down the Irish Sea".

Cabinet members agreed the UK "cannot be kept in the backstop indefinitely" and ministers had discussed "the need for a mechanism to clearly define how that backstop will end".

Brussels watchers say any mechanism controlled by the UK would be anathema in Brussels. Mujtaba Rahman, of Eurasia Group, told The Guardian: "It's simply inconceivable that the EU will hand the UK the right to exit the backstop at a time of the UK's choosing."

The EU's backstop would see Northern Ireland remain in the single market and the customs union until the UK comes up with a different idea, but Theresa May's backstop would see the whole UK stay in the customs union for a limited time after the transition period.

If there's no backstop breakthrough, there's no withdrawal agreement or transition period either, which would be a so called no-deal. There is speculation the post-Brexit 21-month transition period could be extended, which could buy the UK more time to come up with a future trade deal that would avoid the need to trigger a Northern Irish backstop.