Making waves: Boris Johnson looks out over the sea during an election campaign trail stop in Saltburn, Yorkshire.

Let's get Brexit done is Boris Johnson's election slogan. The implication is that, once he gets a working parliamentary majority to ratify his revised withdrawal treaty, Brexit will be quickly done and dusted. This is over optimistic.

There are three realistic outcomes to the election: a Tory majority, a Labour-led government with the support of other parties, or a parliament in which no one can command a majority and form a government.

Even if Boris Johnson wins a majority, he will have to conclude a very complex trade negotiation with the EU, within an almost impossibly tight, self-imposed timeline by December 2020.

He has committed to Nigel Farage that there will be no extension of the December 2020 deadline for the post-withdrawal transition period. This was the basis on which the Brexit Party withdrew its candidates in all Tory-held constituencies.

Reneging on that promise would be costly for Boris Johnson, especially as it would extend the period in which the UK would have to continue contributing to EU funds.

If he does change his mind and decides to look for an extension of the post- withdrawal transition period beyond 2020, he will have to do so by July of next year. The withdrawal treaty (Article 132) only allows for one extension of either one or two years. Incidentally, this is different from Article 50 extensions, to which there is no legal limit.

If the deal is not done before the end of the (very short) transition period, then the UK crashes out of the EU with no deal at all. The implications of this for Ireland and for the UK itself would be grave. This is a real possibility and it is not being discussed enough.

This is only one scenario. The other is what happens if Boris fails to get a majority. Obviously if he fails, the next steps will be decided by a replacement government.

But who will head such a government, and what will be its Brexit policy? Neither question can be answered at this stage. It is unlikely that Jeremy Corbyn's Labour Party can have a majority on its own. Labour might be able to form a majority with support from the Scottish National Party, in return for a pledge to hold a referendum on Scottish independence. Or an arrangement might be made with the Liberal Democrats, but they would want a prime minister other than Jeremy Corbyn. That could happen. If a majority of MPs said, in writing, that they wanted as prime minister a named individual who was not a leader of a party, Queen Elizabeth would call on that individual to form a government.

Either of the Labour-led alternative governments would hold a referendum on Brexit. It might also seek amendments to the existing withdrawal treaty before holding that referendum.

This process would take a year or more to complete, so a lengthy extension of Article 50 would have to be sought. Meanwhile the UK would continue to contribute to EU funds. All this would be quite messy, but it would be infinitely preferable to a crash-out, no-deal Brexit a year from now. A crash-out at the end of 2020 is probable if a Tory government makes unrealistic trade demands of the EU.

A third possibility is that no potential prime minister could be assured of a majority in parliament. Given that the UK now has a five-party system, rather than the two-party one it had for the past century, this is a real possibility. The Fixed-term Parliaments Act requires another election if, after a no-confidence vote, no other government can secure the confidence of parliament within 14 days.

But let's acknowledge that, at the moment, the most likely outcome is a Tory majority government that proceeds to implement the revised withdrawal agreement and negotiate a free trade agreement (FTA) with the EU.

Given the likely composition of the new Tory parliamentary party, the UK negotiating position on the FTA could be very difficult for the EU to accept. Some Tories might even prefer no deal.

Before discussions begin, the EU side will have to secure a negotiating mandate from the 27 member states. This will not be easy. Many states will have sensitive issues vis-a-vis the UK, for example:

:: Fisheries for Spain,

:: Agriculture for France,

:: Rules of origin for all members, and crucially,

:: The maintenance of a level playing field for competition between firms inside the EU and those in the UK.

Boris Johnson has said that, for him, the UK being able to have different environmental, social and product standards is the "whole point" of Brexit.

There are real fears the UK would try to undercut the EU in these fields.

So the EU will demand firm justiciable guarantees in the FTA that this will not happen.

Likewise the EU will want justiciable guarantees that the UK will not give subsidies to its industries of a kind that would not be permitted in the EU.

The EU will demand binding arbitration on these issues, but that is an allergic issue for Brexiteers.

If that is not agreed, it is hard to see how the EU could give up the possibility of introducing tariffs on UK exports to the EU, to level up the playing field.

Similar problems arise for agriculture and fisheries. There will also have to be a negotiation about co-operation between the UK and Europol and on money laundering. The earliest that the two sides would even be ready to start negotiating these difficult questions would be March 2020.

It is hard to see how it could all be wrapped up by December of next year.

Irish Independent