Article 50 emerged 15 years ago, in a convention of 200 parliamentarians from all the countries who then were members of, or were then negotiating to join, the EU. I was their Secretary-General. One of their concerns was to demonstrate that the Union was a voluntary partnership of sovereign nation-states, based on treaties between states, not the incipient super-state of Eurosceptic nightmares.

Including an Article setting out a procedure for orderly divorce was one of several ways of underlining the voluntary nature of the Union, and I was its author. I'm certain no-one dreamed that in 2017, a member state would trigger the procedure, as Mrs May did on 29 March this year. But now that we're in the procedure, it's important to understand it; and I am concerned that some aspects of the Article seem to me rather inadequately reflected, or indeed misinterpreted, in our current public debate. This is the argument I made in a speech hosted by the Open Britain campaign today.

First, and crucially, as required by the Treaty, May's letter was only a notification of the UK's "intention" to withdraw. Intentions can change. We still have all the rights of a member-state, including the right to change our minds and our votes, as member-states frequently do, for example after elections. The Article is about voluntary withdrawal, not about expulsion: we don't have to go if at any stage, within the two years, we decide we don't want to.

The clause that says that "once we're out, we're out" says just that, and only that. If we had wanted an intention to go to be the Rubicon moment, if we had wanted a notification letter to be irrevocable, we would have drafted the clause to say so. But we didn’t, and the clause doesn’t. So, the die is not cast irretrievably. The letter can be taken back.

That has subsequently been confirmed by formidable legal experts. Let me cite just two. Jean-Claude Piris, Legal Counsel to the Council in my Convention days, is clear that “even after triggering Article 50, and notifying the EU of its intention to leave, there is no legal obstacle to the UK changing its mind." Sir David Edward, UK Judge in the European Court of Justice when the Article was drafted, says the same.

The government gives the impression that the Rubicon has been crossed, but it refuses to publish the legal advice they have received on the subject. I think we know why. The government has been careful not to say that we could not take back May's letter, because it knows that we could if we wanted to. The fact is that a political decision has been made, in this country, to maintain that there can be no going back. Actually, the country still has a free choice about whether to proceed. As new facts emerge, people are entitled to take a different view. And there's nothing in Article 50 to stop them. I think the British people have the right to know this – they should not be misled.

Supposing we were to exercise our right to withdraw May's letter, how would leaders across the Channel react? We know from what they have said: they would applaud. Let me cite a couple of Presidents. “If the UK wanted to stay, everybody would be in favour. I would be very happy.” That's Antonio Tajani, President of the European Parliament. “It is in fact up to London how this will end: with a good deal, no deal, or no Brexit.” That's Donald Tusk, President of the European Council. Or take the Taoiseach, Leo Varadkar: “The door remains open for the UK to stay in the EU." Yes. It does. And President Macron has said the same.

Most EU leaders think Brexit would be a disaster, worst for us, but bad for all. Most believe that, in a world of Donald Trump and Vladimir Putin, of Daesh and Islamic State, of Asian competition, of climate change and migration misery, Europe should stick together and work together. If we were to change our minds, Putin and Trump would be disappointed, but our near neighbours, and our true friends across the Atlantic and in the Commonwealth, would cheer. I think the country should know that.

My second concern is less fundamental, but I am uneasy that the country isn't being told much about the possibility of taking more time. I don't know why May was in such a rush to send her letter in March, before her Cabinet had an agreed plan. And I don't know why both government and Opposition now seem to discount the possibility of our seeking an extension. If, for example, we were to need time for Parliament to consider a final deal, or to check that the country, having seen the facts emerge during the negotiation process, still wanted to Leave, I do not see any of 27 democracies denying us the chance to consult the people.

My last point can be briefly put. I think the country should be aware of one big difference between, on the one hand, negotiating for accession, and, on the other, drawing back from secession: in the former, there's a price to pay; in the latter, there isn't. While we're in, we're in; and there would be no price to pay if we were to decide to stay in. But if we later decided to apply from outside, to return, the budget rebate would have gone.

My conclusions are simple. The national debate about Brexit should take account of the facts that our Article 50 letter could be withdrawn without cost or difficulty, legal or political. While still in, we also have the option of stopping the clock, in order to consult the people again. But once out, there is no easy way back in. All these facts will still be relevant when Parliament next autumn gets the chance, as it must, to assess the outcome of the negotiations.

Lord Kerr was the Secretary General of the European Constitutional Convention, which drafted Article 50, between 2002 and 2003. He is a leading supporter of Open Britain.