Senior EU sources fear a looming crisis in the Brexit negotiations in the coming weeks because of British Prime Minister Theresa May's growing incapacity to act due to mounting pressure from both Brexit hardliners and pro-European Tories over the issue of the customs union.

One senior official has told RTÉ News that hardline elements in the Conservative Party would be prepared to abandon the UK's commitment to the Irish backstop, the guarantee of no hard border agreed in the Joint Report last December.

Mrs May's preferred option for solving the Irish border problem, through a so-called customs partnership, has been rejected by senior cabinet Brexiteers.

The partnership model would only have been considered by the EU if it meant Britain staying de facto in the customs union for years to come, and remaining aligned with the rules of the single market.

However, that option is off the table for Mrs May because of the threat posed by hardliners.

Boris Johnson attacks 'crazy' customs proposals

Ireland and the EU Task Force want a breakthrough on the Irish backstop by June.

However, Mrs May is clearly not in a position to agree to a text that would give the backstop operational legal effect.

One senior official described a scenario where the opposition Labour Party would not support October's withdrawal agreement, meaning Mrs May would have to rely on hardline eurosceptics and the DUP.

In that scenario, it would be near impossible for her to agree the backstop in a way that was acceptable to the Irish Government.

There is even speculation at the highest level that Mrs May could be forced to seek an extension to the two-year Article 50 process, a move that would leave the Brexit process in considerable disarray.