Should remainers now “get behind” Brexit and turn their energies to shaping a close UK-EU relationship or to rejoining the EU post-Brexit? That is what some respected Brexit commentators have started to accept. The essence of their point is that all the legal routes to stopping Brexit are now politically too unlikely and that, since the mandate from the referendum was only to leave the EU, all that happens afterwards is up for grabs.

But for remainers to give up on the campaign to stop Brexit would be wrong on principle, and practically ineffective.

A key lesson from my time as a negotiator for the UK in the EU is that it is essential to be calm, at full strength, and there up until the very last moment of the process. The final stages are when events happen at an artificially accelerated rate, and realities can change in an instant. Slinking out early, or switching focus to the next issue, opens you up to missing a game-changing opportunity. While time may be tight, a week is now an eternity in British politics.

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On principle, we should oppose what we know to be harmful. We know that Brexit will harm our families, friends, neighbours, fellow citizens, and the country and its standing in the world. To acquiesce quietly to this would be wrong while there is some chance of success.

The odds are against stopping Brexit. They always have been, but it can still happen. The main obstacles are political, not legal. For example, were the Labour party leadership to significantly alter its position, the odds could shift considerably in favour of remaining.

It is true that, without a request, and EU27 agreement to an extension of the two years allowed by article 50, or a revocation of the notice given under it, the UK will leave the EU on 29 March 2019. Time is tight, but it was always going to be, and there are good reasons to believe that the EU27 would grant an extension for democratic processes in the UK to play out. Successive statements by EU leaders have made it clear that, for the EU27, the UK abandoning Brexit is still a preferred option.

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There was a mandate from parliament for the government to invoke article 50, and negotiate terms, but this does not mean that whatever those terms are must be accepted. By the time the withdrawal agreement and a declaration on the shape of a future relationship is finalised (if indeed they are finalised in time, which is far from certain) it will be well over two years since the referendum. Their contents were not and could not have been known at the time.

If Brexit goes ahead, we would then face negotiations on agreements on a future relationship during a transition period, but with no option to remain, and only the option of a disastrous no-deal cliff edge if those results were unsatisfactory. At that point, four and a half years after the referendum, whatever is on the table would have to be accepted.

As a vote, either in parliament, or if it wishes through a referendum, requires an alternative other than certain disaster to deserve the term “meaningful”, the withdrawal agreement represents the last opportunity. Should parliament reject it, either itself or through a referendum, it would have every right to require the government to withdraw article 50 notification.

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The existing mandate is weakened by allegations that the small majority in the referendum was gained on the basis of deliberate untruths and potentially serious wrongdoing. Had there been an open, frank and (largely) honest debate, based on truthful, transparent campaigns from both sides, and resulting in a large, decisive majority, it may not be so weak. That a general election was called with the express purpose of strengthening the government’s mandate, and resulted in its majority being eliminated weakens its mandate yet further.

It is therefore reasonable to consider the government’s mandate to be limited to the invoking of article 50 and the negotiation of a withdrawal agreement, and not to require the automatic acceptance of any outcome that is presented. It is up to parliament to either issue, or seek a public vote to issue, a mandate to accept the outcome or seek to remain a member of the EU.

In practical terms, there is little point in “getting behind” Brexit and focusing instead on the post-Brexit world.

There is little point in 'getting behind' Brexit and focusing instead on the post-Brexit world

Remainers’, or indeed anybody’s, ability to influence the government’s approach has been negligible. Business leaders, senior civil servants, devolved governments, renowned economists, soft leavers, noted experts, and even some more moderate cabinet members have been systematically ignored. The government has not listened, and there is no evidence that it would begin to, if we accepted Brexit. It is far more likely that it would simply pocket our move, use it to claim support for its misguided approach, and continue as before.

The same goes for the official opposition. Giving up opposing Brexit would be seen as an endorsement of its woefully ineffective triangulating approach. It would lessen, not increase, the pressure within Labour to genuinely oppose Brexit.

Our future options would also be very limited. The government has expressly ruled out the closest possible relationship available. Rejoining is a purely long-term prospect. After the hassle, uncertainty and cost that Brexit has caused, it would be impossible for EU27 leaders to sell an early UK re-entry to their own electorates, even in the unlikely event that they wanted to. Most in Brussels put the earliest point for rejoining at least a decade from now, and many say longer. Brexit really does mean Brexit.

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The government and much of the media have already pre-trailed the myth that Brexit would have been a roaring success had it not been for those pesky foreigners, judges, civil servants, businesses, economists, remainers. If this is allowed to stand, it is easy to imagine the UK a few years down the line, dealing with the damage caused by Brexit to its society, economy, political culture and constitution, in which all its ills are blamed on the myth of stab in the back by the EU. In that case, even simple agreements on cooperation may be impossible, and every dispute would be blown up into a terminal crisis. This would be appalling for the UK, the EU and the world as a whole.

Seven or eight months ago, those like me calling for a referendum on the deal and arguing that Brexit could be stopped were dismissed by many as cranks and fantasists. Now we have MPs from all major parties arguing for one or both, and both being discussed as real possibilities, even by the overly cautious BBC.

So remain campaigners are having an effect. It may not be enough, and the odds are always stacked against us. Even if you disagree with the principled objections to giving up, there’s no payoff, no practical benefit from doing so, and you can bet your shirt that it would be used against us.

• Steve Bullock, a former negotiator for the UK in the EU, is a campaigner, commentator and writer on Brexit

This article was amended on 6 April 2018 to remove a misplaced link