Henry Newman is Director of Open Europe.

As ConservativeHome readers may recall, I backed the Withdrawal Agreement, not because it was perfect, but because I was convinced it offered the surest way to deliver Brexit on time. I wouldn’t have started from there but – as you often hear on Whitehall – “we are where we are”. I also felt that some critics of the deal exaggerated its negatives, while ignoring some of its advantages. Others seemed unrealistic about the actual alternatives available after nearly three years of negotiations.

Leading Brexiteers including Boris Johnson, Gisela Stuart, Michael Gove, Jacob Rees-Mogg, Dominic Raab, Andrea Leadsom, Norman Lamont, Michael Howard, and Iain Duncan-Smith all ultimately reached the conclusion that the Withdrawal Agreement was the least bad option and backed it. Yet, as everyone knows, the deal failed to pass on its third and final attempt.

It’s overwhelmingly likely that later today Johnson will secure the Conservative leadership. And in my judgement, although both candidates have strengths, he stands a better chance of successfully delivering Brexit than Jeremy Hunt. He will however face an extraordinarily difficult inheritance – a party bitterly divided, where opinions have polarised over the past three years; an extremely weak Parliamentary position, made still worse by defections; and a Party with two factions both threatening to bring down a Government which pursues a Brexit policy with which they disagree. He is likely to have one of the shortest honeymoons of any recent leader.

Johnson’s premiership will be defined – do or die – by what happens in his first 100 days, in the run up to 31st October. There are four broad possibilities, only two of which would mark a success for him.

First, extend Article 50 again. This would require unanimous agreement by all EU member states and the UK. Although Jeremy Hunt left open the possibility of extension, Boris Johnson has categorically ruled it out. So an extension is only likely if the Executive is forced by Parliament. Without an extension there is not enough time to run a second referendum.

Second, revoke Article 50. This is the nuclear option – opposed by almost all Conservative MPs. Most MPs opposing Brexit instead argue for a second referendum, which could ultimately lead to revocation. That of course would entail a further extension. However, it remains possible that in a forced choice between No Deal and Revoke shortly prior to 31 October, a majority of MPs might choose to revoke.

Third, agree a deal. The current deal will not pass the Commons. So the only possibility is a revised deal that has changed enough to allow MPs to reconsider their position.

Fourth, leave the EU without a deal. This remains the legal default in the absence of any other option being agreed. A No Deal exit would mean significant short-term disruption for both the EU and UK, but over the medium-term the economic effects would be limited, as previous Open Europe research has explored. However, a No Deal is opposed by a majority of MPs who will try every parliamentary trick going to block it from happening.

The surest path to Brexit would be a new deal with the EU. There is still a desire in European capitals to reach agreement with the UK. But there’s also profound frustration with the UK. Over the last three years patience and trust – in both directions – has worn thin. Reaching agreement will require compromise on both sides. Johnson will need to charm and reassure not just Berlin and Paris, but above all Dublin.

There is little chance of Brussels junking all the existing Withdrawal Agreement, and starting from scratch. Anyway, the majority of the text is broadly acceptable, as the ‘Brady amendment’ demonstrated. Some in Westminster appear to believe the EU could agree to things that Brussels has shown almost no inclination towards. EU member states won’t agree to replace the withdrawal agreement with just a bare bones deal under Article 24 of the GATT. They also won’t sign off on a standstill transition, outside the jurisdiction of the European Court, which isn’t associated with the Withdrawal Agreement.

The Commission’s negotiating team are, however, quietly considering the possibility of surgical changes. But agreeing to any changes will depend on how negotiations with the new Prime Minister proceed, and would require a revised mandate from the European Council. There’s a desire from some key EU capitals to find a way through, but there’s a limit to how much Brussels will move at this late stage. It’s not just the UK side of the negotiation table which has politics to address, pride to preserve, and interests to defend.

Some MPs may balk at the idea of only securing surgical changes to the legal texts. But surgical changes could entail more of a heart transplant than an appendix removal. What matters ought to be the extent to which changes affect the fundamental substance of the deal. That’s why I was so surprised to hear both leadership contenders carelessly telling Tom Newton-Dunn they would reject the idea of a Brexit deal with an exit from the backstop. In fact, if they secured anything like that it would be a huge victory. The litmus test ought to be whether the Attorney General’s legal advice can be changed.

The prime concern of most Brexiteers was a fear that the Withdrawal Agreement could create – via the backstop – an essentially permanent arrangement where the UK would lose the ability to develop an independent trade policy, and would be subject to EU rules and the jurisdiction of the European court. If that concern can be addressed, many of the worst fears of the Withdrawal Agreement should fall away.

The excellent work of Prosperity UK’s Alternative Arrangements Commission reveal the sort of solutions which both sides could adopt to replace the backstop.

At the same time, the EU are open to a significant re-write of the Political Declaration. A new prime minister should secure agreement that the UK and EU will negotiate a new relationship modelled on a Canada-style free trade deal, rather than Chequers. The document could also provide further reassurance that the UK will secure an independent trade policy, as well as autonomy on foreign policy, security and defence matters.

Now, nearly four months after the UK was supposed to Leave, Brexit seems more uncertain than ever, precisely as I feared earlier this year. There is a path through to securing Brexit, but it’s a narrow one. Either side lie dangers: a general election before Brexit, or a second referendum. In just a hundred days, we will know whether Boris Johnson has succeeded.