It is two years since the British people voted to leave the European Union, setting in train the most momentous constitutional upheaval of the post-war era.

What has been achieved in that period?

Theresa May formally invoked Article 50 in March 2017, marking the start of the two-year countdown to the UK’s departure from the EU and paving the way for the start of negotiations on the terms of British withdrawal.

In December 2017 she achieved a provisional agreement covering Britain’s £39 billion “divorce bill”, future citizens’ rights and the border between North Ireland and the Republic. Last March, EU leaders gave the go ahead for talks on Britain’s future relations with the bloc, including a potential trade deal, to begin.

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Sir Tim Barrow hand delivers Prime Minister Theresa May’s Brexit letter under Article 50 of the EU’s Lisbon Treaty to EU Council President Donald Tusk (Alexander Britton/PA)

In June, the Prime Minister secured the passage of her flagship EU Withdrawal Bill largely unscathed despite threatened backbench revolts.

So is the Government united on the way forward?

Hardly. Ministers are divided over what sort of future relationship they want with Brexiteers like Foreign Secretary Boris Johnson pressing for a clean break while Remainers like Chancellor Philip Hammond want to keep as close as possible to the EU.

So far Mrs May has largely been able to keep them together, largely by putting off the most difficult decisions, but a planned away-day in July for the Brexit “war cabinet” intended to thrash out their differences could become a new flashpoint with unpredictable consequences.

The Prime Minister also faces further disruption in Parliament, with pro-EU Tories plotting to amend the forthcoming trade and customs bills.

What are the main stumbling blocks?

The Irish border issue, which still has to be finally resolved, has come to crystallise all the most difficult problems in the Brexit negotiations.

All sides agree that there should be no return to the “hard border” of the past but there is no agreement as to how that can be achieved.

The EU says Britain’s decision to leave the single market and the customs union means border checks will be necessary unless Northern Ireland retains the same customs and regulations as the EU.

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The border between Londonderry and Donegal at Bridgend (Deborah McAleese/PA)

The Government, however, says that would effectively meaning imposing a border between Northern Ireland and the rest of the UK, something that is an anathema to both the Conservatives and their DUP allies.

Meanwhile Britain’s favoured solution of using technology to avoid the need for physical border checks has been dismissed as “fantasy island” in Brussels. Deadlock.

What happens next?

The Government says it wants agreement on all the withdrawal issues by the time of the October EU summit, however there are severe doubts that that can be achieved. The next opportunity for agreement would be the December summit but that would leave the rest of the Brexit timetable looking extremely tight with ratification required from both the UK and European parliaments.

Will Britain leave the EU on March 29 2019?

The Government says it will and Labour is also committed to taking Britain out of the EU, despite significant unrest on the back benches. However the path ahead is far from clear is no certainty that ministers will get the Brexit deal they are seeking. Watch this space.