By the time this column is published, the Prime Minister is expected to have triggered Art 50 of the Treaty on European Union by notifying the European Council of the UK’s intention to withdraw from the EU. Ministers had assumed that notification would be done by the Prime Minister exercising prerogative powers. But following the decision of the Supreme Court in Miller it was the UK’s sovereign Parliament at Westminster that handed Theresa May the power to do so.

After the House of Lords took 44 hours to add two amendments to the European Union (Notification of Withdrawal) Bill, the Commons needed only two hours to send the Bill back to their Lordships. The Commons rejected a Lords amendment that would have given Parliament an effective veto on May walking away from Brexit negotiations without a deal. They also rejected the Lords amendment that would have guaranteed the rights of EU citizens currently living in the UK after Brexit, despite Keir Starmer QC’s best and eloquent efforts for the Opposition. Among the pro-European Tories to defy the three-line whip by abstaining were former Attorney General, Dominic Grieve QC and current Chairman of the Justice Committee, Bob Neill.

Notification marks the beginning of the formal process of disengagement from over 40 years of treaty obligations which still bind the UK to the EU. It triggers the opening of negotiations with the European Commission which are intended to lead to a withdrawal agreement to be concluded, in 2019, by the EU Council by qualified majority (72% of participating states, excluding the UK), after having obtained the consent of the European Parliament. Time is tight. The task is enormous and complex. The negotiating period can be extended but only if all the 27 remaining member states agree.

The UK’s withdrawal is without precedent. Until now only Algeria (upon independence from France) and Greenland (which remained part of Denmark) have left the EU. Both events happened before the establishment of the Art 50 process.

No one can safely predict how the Art 50 negotiations will proceed. By the time the UK exits the EU it is essential that there is clarity about the institutional and financial consequences of leaving the EU (including the resolution of all budget, pension and other liabilities and the status of EU agencies currently based in the EU). In addition border arrangements between Northern Ireland and the Republic of Ireland will need to be settled and institutional arrangements agreed for north-south cooperation and east-west cooperation underpinning the Good Friday Agreement.

The status of UK citizens living in the EU and status of EU citizens living in the UK will need to be agreed. The UK’s continuing relationship with EU regulatory bodies and agencies and the status of ongoing police and judicial cooperation will need to be clarified, together with the UK’s participation in Common Foreign and Security Policy missions. A clear framework for UK-EU trade in goods and services (including legal and related professional services) will be needed, as will clarity on the location of former EU powers between the UK and devolved governments. In parallel with the withdrawal negotiations designed to clarify the terms of the divorce settlement, the UK will need to negotiate future trade relationships with the EU and with the rest of the world.

Dealing with such an array of multiple, simultaneous challenges will present Whitehall mandarins with the biggest set of challenges they faced since the task of planning and coordinating a programme of national reconstruction at the end of the Second World War. The pressure on the Department for Exiting the EU in particular and on other departments with responsibilities to ‘deliver Brexit’ will be considerable. Following a recent period of brutal headcount reductions in Whitehall (including at the Ministry of Justice), the size of the Civil Service is likely to increase in order to plug gaps in knowledge and expertise, including in law, economic diplomacy and trade policy. Permanent Secretaries as well as the Treasury Solicitor as Head of the Government Legal Department need to address the capacity-building challenge rapidly.

It is possible that no deal is struck at the end of the withdrawal negotiations. Indeed, the government has repeatedly said that it will walk away from a ‘bad deal’ which means that preparing for no deal is all the more essential. Hopefully the Art 50 negotiations will be successful. The UK and the EU have a shared interest in reaching agreement but with the possibility that negotiations could fail, the Commons Foreign Affairs Committee has argued that each department needs to have a contingency plan. The main legal and technical issues that would be unresolved if the UK left the EU with no deal were in place are examined in a recent report by the Commons Foreign Affairs Committee which draws on wide-ranging and extensive evidence submitted by the Bar Council.

A number of departmental select committees have been finalising their reports to inform parliamentarians about the issues that will be raised by the withdrawal negotiations. These include the Lords EU Committee’s Civil Justice Sub-Committee following their ‘Brexit: Civil Justice Co-operation and the CJEU’ inquiry, to which the Bar Council gave written and oral evidence; and the Commons Justice Committee which conducted an inquiry into the likely effects of Brexit on the processes of criminal and civil justice, as well as on the financial effects on the legal sector, business and the economy more widely, to which the Chairman of the Bar, Andrew Langdon QC gave evidence. These select committee reports should not only help parliamentarians understand the progress and significance of the withdrawal negotiations but also sharpen their weapons in exercising their scrutiny of legislation following a withdrawal agreement.

There can be little doubt that Parliament’s ability effectively to scrutinise the ‘Great Repeal Bill’, and the vast mass of related legislation (primary as well as secondary) is going to be severely tested. The government’s commitment to publishing a white paper on the Bill (desirably with draft clauses) is to be welcomed. The government will need to take the lead in setting out for Parliament exactly what will be required to preserve the effect of EU law following Brexit. The final shape of that law will of course depend on the outcome of the UK’s negotiations.

The process of converting the corpus of EU law will involve two stages: the first will be the preservation of EU law and converting it into UK law (subject to amendment to work in the UK context); and secondly, in the longer term after Brexit, reaching agreement between government and Parliament about (what was) EU law which should remain part of EU law. The Great Repeal Bill will facilitate the first stage. Separate legislation will be needed to accomplish the latter.

Parliament will need to strike an appropriate balance in granting the government relatively wide delegated powers for these purposes whilst ensuring that they cannot be used to implement new policies desired by the government in areas which were formerly within EU competence.

In a recent and powerful report, by the Lords Constitution Committee to which the Bar Council contributed evidence, the prospect of a ‘massive transfer of legislative competence from Parliament to government’ was identified. The volume and complexity of secondary legislation likely to be laid under the Great Repeal Bill as well as under other primary legislation related to Brexit, will put significant strains on Parliament’s resources. This raises constitutional concerns of a fundamental nature. Parliament needs to consider how best to use the extensive delegated powers that will be sought, recognising that the Great Repeal Bill will be an exceptional piece of legislation, necessitated by the extraordinary circumstances of Brexit.

Nevertheless the exceptional and extraordinary circumstances constituted by Brexit should not be taken in and of themselves to be a sufficient answer to legitimate concerns relating to the proper balance of constitutional authority between the Executive and the Legislature.