Brexit is the most important political event for the UK since the Second World War, and will shape the UK economy for generations. But this is completely unknown territory for all parties concerned.

But though triggering Article 50 takes us into a landscape with so many unknown unknowns, the one thing many commentators agree on is that the current timeframe to conclude negotiations is unrealistic – particularly if a “mixed agreement” is the result, as it would require ratification by all 27 national parliaments within the European Union.

On the EU side, the political landscape of raising populism will be problematic. They will be mindful that a successful Brexit would present a threat to the European project. They also know that hard-headed negotiations may not begin until after the French elections in June, and further important discussions deferred until after the next German government is formed in late autumn. That would leave the UK with a negotiation period of approximately 10 months.

The best-case scenario appears to be one in which the UK and EU 27 find a way to conclude a transitional deal without a lengthy sign-off process: tricky but not impossible. The European Economic Area (EEA) Treaty might provide a template, but it is likely that the UK would have to accept budget contributions, some pooling of sovereignty and a compromise on freedom of movement to achieve it. And even such if transitional provisions are desirable, then how do we get there? It’s not easy. Article 50 contains the basis for a withdrawal agreement, not the full process or any other alternatives. Negotiating a transitional agreement may even require ratification from some small member states such as Wallonia.

The truth is that Article 50 of the Lisbon Treaty was designed specifically so that no member state would contemplate triggering it. And yet that is what we are doing.

This is what will happen once Article 50 is triggered

It raises far more questions than we have answers available. For example, is it realistic to imagine the EU 27 ever agreeing a wide-reaching new relationship with Britain in an eighth of the time it took to conclude the recent free trade agreement between Canada and the EU? Even if it was, if the Court of Justice of the European Union (CJEU) assess the nature of the various transitional agreements to ensure they are lawfully adopted (as we may expect), how long might that add to the Brexit timetable? And would the CJEU inevitably be the final arbiter of compliance with these agreements? Would the UK ever accept that?

The UK’s ability to conduct third-country free trade agreements – something Theresa May and David Davis have both spoken about with enthusiasm – is a red herring, as the primary focus of Brexit negotiations will be to conclude a trade deal with the EU. Only once this is achieved, only then would the attention of the Government be turned to the rest of the world.

Economically, the UK could risk seeing huge disruption to existing business models if there was to be a sudden exit from the customs union in March 2019. The reality is that business cannot wait for the white smoke of a secret deal to emerge like the election of the Pope.

People like myself – calling for a transparent, scrutinised Brexit negotiation process to ensure it is not executed at the cost of Parliamentary sovereignty – are accused of wanting to tie the Prime Minister's hands. But it is the Eurosceptic Conservative MPs and parts of the press that are pushing for a clean break without any safety nets, that are constraining May’s ability to compromise.

The more we understand the process ahead and its timing, the more we can plan. Not knowing, and not expecting to know anything, is likely to result in a default conclusion to plan for the worst case. For example, it is in the UK and EU’s interest to seek short-term regulatory equivalence for financial services, even though it will be unsustainable over the long-term due to regulatory differences and the competitive risks the City would pose for the eurozone.

A hard Brexit is avoidable. A very broad agreement on a new relationship between the UK and EU 27, combined with a transitional deal, is possible before March 2019. It will be an uphill struggle, due to the complexity of negotiations, political risks and most crucially time constraints, but it is possible.

If forging a new relationship proves impossible and the UK leaves with no deal, many of those involved in the negotiations may well feel they are in need of a very well-earned break. But if they fail to get us the agreement we need, they may find that more difficult than they expect. Their flight out of Britain would rely on pilots’ licences, air safety certificates, landing permissions – all reliant on EU regulation and laws, from which would have in effect opted out. All international landing agreements have been brokered by the EU. Each of these would have to be negotiated separately with each country – all of which will want to see either a transitional arrangement, or that we have set up our own safe and reliable regulator in time. That sums up the scale of the problem.

In view of the realities, of the all the unknown unknowns, I sincerely hope all parties in the Brexit negotiations resist the temptation of deliberately creating conflict that will add to the already mammoth task ahead.