I first witnessed the power of a big, borderless scientific community when, as a young PhD graduate, I swapped Oxford for the mountains of Colorado.

There, I?saw the beginnings of one of the most important scientific developments of my lifetime: the discovery and the production of the Bose-Einstein condensate (BEC). I was struck by the fact that the exchanges of ideas that led to this discovery did not occur in scientific papers or peer-reviewed reports. They happened via the free movement of people across the 50 states of the US, from sea to shining sea.

Those who started as undergraduate students at the Massachusetts Institute of Technology and became doctoral students at Stanford University joined with others from everywhere in between to become faculty and scientific leaders at the University of Colorado Boulder, their thoughts cross-pollinating as they buzzed around the physics department. They coalesced from far and wide into a team that ended up in Stockholm, to receive the ultimate scientific prize.

Of course, the US¡¯ national scientific renown draws in even more talent from around the world; overseas scientists played a major role in many of the Nobel prizes showered on the US over the course of the 20^th century. But the importance of the country¡¯s vast native gene pool in attaining and maintaining that renown should not be underestimated. My experience convinced me that it was every bit as important as a generous funding environment and great institutions.

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When, years later, colleagues at a European Science Foundation conference asked me why it had been the US that made a BEC first, I told them all this. I noted that the great scholarship of Europe flourished centuries ago, when there were no passport requirements to stop scholars travelling from Cracow to Oxford, via Heidelberg and Paris. And I said that, with the European Union, we were recreating those conditions, after all the fracture of two terrible wars and decades of ideological enmity. As our conference ended, we ate and drank and boisterously sang our national anthems, buoyed by the knowledge that we were, in fact, bonded together in a common endeavour to abolish scientific nationalism.

I was proud that we were building a new way of doing collaborative research that rivalled the opportunities on the other side of the Atlantic, through schemes such as Marie Curie fellowships, transcontinental projects and, above all, allowing scholars to move freely for study and work.

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In 1989, at the European Organisation for Nuclear Research, a software engineer from the UK invented the World Wide Web. On a former airfield in Oxford, I worked with a consortium comprising 30 fusion research organisations and universities from 28 European countries to host the world¡¯s largest magnetic fusion experiment, the Joint European Torus.

The UK has had more benefit from this European Union of Scholars than any other country ¨C and not only in science. We have seen the universities of Oxford and Cambridge become the MIT and Harvard of Western Europe, thanks to an influx of talent from across the continent. Indeed, many talented European scientists have made their homes in cities across the UK, building international research groups that allow the country to punch far above its weight in academic terms.

And, just as I saw in the US, the excellence fostered by our free European collaboration has attracted even more talent from nations further afield, such as China, India ¨C and even the US itself.

But while we have risen to many scientific challenges together, there are so many more that urgently need our shared insights and purpose. Erecting barriers to free movement, as Brexit threatens to do, will seriously undermine our ability to respond.

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This is what is at stake: one of the greatest scholarly communities the world has ever known, and the opportunities for children born under whatever flag to join an enterprise more important than the petty jealousies of nationhood.

No stopgap funding will make up for the loss of the European Union of Scholars. That is why, as the deadline for a Brexit deal looms, I beg the UK government to do all in its power to preserve these precious bonds of learning.

Sir Keith Burnett is the outgoing vice-chancellor of the University of Sheffield and president of the UK Science Council.?