Question of the Irish border remains a major hurdle as London and Brussels seek to agree the terms of UK withdrawal.

UK Prime Minister Theresa May has said Britain’s divorce deal with the European Union is 95 percent settled but admitted the problem of the Irish border remained a “considerable” sticking point.

Addressing the British parliament on Monday, May said that the United Kingdom might ask for a short extension of a planned transition period to ease its exit from the European Union, or Brexit, which is due to take place on March 29, 2019.

“There are some limited circumstances in which it could be argued that an extension to the implementation period might be preferable if we were certain it was only for a short time,” May told legislators in London.

The prime minister was updating parliament on a summit of EU leaders in Brussels last week, where she attempted to make progress on an agreement on the terms of the UK’s withdrawal from the bloc.

She reported progress on some issues including in areas of security, defence and Gibraltar, noting that “95 percent of the withdrawal agreement and its protocols are now settled”.

Gibraltar, a British overseas territory located at the tip of the Iberian peninsula and regularly claimed by Spain, will leave the EU along with the UK.

The hardest five percent

But May also admitted that the question of how to avoid a so-called “hard border” between Ireland and Northern Ireland remained a major, unresolved challenge.

“There is one real sticking point left, but a considerable one,” she said, referring to the Irish question.

When Britain leaves the bloc, some form of border will have to be drawn between Northern Ireland – which is part of the UK – and Ireland – which is an EU member – to allow for customs controls on goods, produce and livestock.

Both the UK and the EU are keen to avoid a hard border, which would be policed and involve stops and checks at crossing points.

The two sides accept that a “backstop” solution is desirable in the case that the UK leaves the EU without a comprehensive withdrawal agreement, although they disagree on what the backstop would entail.

The EU has proposed a backstop that would involve Northern Ireland remaining in the EU customs union and large parts of its single market.

This solution would effectively re-draw the border between the UK and EU in the Irish Sea, an outcome that London is keen to avoid.

May has proposed that the whole of the UK should remain in the EU customs union. Brussels is currently considering this but has not been prepared to accept the proposal so far.

The EU has also volunteered to extend the post-Brexit status-quo transition period by one year. Currently, the transition period is due to close at the end of 2020.

May said last week that she was open to an extension, a move that drew fierce criticism from members of her Conservative party.

On Monday, she reiterated the position, stating that the UK might need to ask for a short extension “as an alternative to the backstop”.

“I have not committed to extending the implementation period,” she said. “I see any extension, or being in any form of backstop, as undesirable.”

Reporting from London, Al Jazeera’s Nadim Baba said May’s position as leader of the party seemed to be in jeopardy, amid rumours of leadership challenges.

“It seems to be getting ever more precarious,” he said. “We are hearing that centrist Conservative MPs are now joining dozens of others who have sent letters to the party chair, asking them to basically trigger a leadership contest.”

Baba added that May is expected to hold a cabinet session on Tuesday, before possibly meeting backbenchers a day later.

“We’ve had rumours of challenges before and they’ve gone nowhere – but the clock really is ticking,” he said.

“Many people say that this is just to add pressure on her not to give in to Brussels in terms of that transition period.”