“A no-deal Brexit would be ‘a disaster’ for Spain, the country’s UK tourism chief has warned” – so began a news story from this week’s Abta Travel Convention in Seville.

Javier Piñanes, director of the Spanish tourist office in London, may have been more candid than his advisers wished. Yet by naming the issues that desperately need to be resolved by 29 March 2019, he has done the travel industry – and holidaymakers – a favour.

On an average day, more than 50,000 British visitors touch down in Spain, far more than any other nationality. Several million of us have already booked places on the playas for next summer. Yet while planes from the UK will be permitted to fly over European Union nations, thanks to the 1944 Chicago Convention, there is no legal mechanism for them to land in the EU.

As Señor Piñanes also pointed out, no-one knows whether British visitors to Spain will need visas, nor whether they will continue to be covered by a reciprocal health agreement.

So what exactly do we know about post-Brexit travel? I can tell you, but you might not like the answers.

Starting in Spain: if you are tempted to visit the Royal Palace in Madrid, best go by 29 March 2019 and get in free with a wave of a European Union passport in the late afternoon from Monday to Thursday. After Brexit it will cost €10. In Paris, the chance for young Brits aged 18 to 25 to visit the Louvre free will also end; that’s €15.

At least a year from now, some of us will be able to wave a smart blue passport – the same shade as used by EU member Croatia – while queueing to pay. The government has promised passports will change colour.

Still, at least Britain’s attractions, free or not, will be emptier. Another “known known” is that it will be more difficult for many Europeans to come to the UK on holiday.

As stated in a leaked Home Office document: “We intend to require all EU citizens to travel on a passport.”

Right now, EU citizens with national ID cards can visit more than 30 foreign nations: the other members of the union, geopolitical oddities such as Andorra, assorted Balkan states and the former Soviet Republic of Georgia.

As a result, levels of passport ownership in countries with ID cards are lower than in Britain.

After the UK leaves, the number of possible destinations for ID-card holders will drop by one. While some European citizens may go through the hassle of getting a passport just to visit the UK, many will not.

Look on the bright side, though: the slump in inbound tourism will coincide with an exodus of the EU staff on whom the UK’s hospitality industry depends. Demand and supply will dwindle in harmony.

That’s about it in terms of certainties. But while I’m here, you can have a couple of “very likelies”: duty-free allowances will return for travel between the UK and the European Union (but no more loading up the boot with burgundy at Calais); and the government says it wants to keep European air passengers’ rights rules in force, despite them being unfair and inconsistent.

And the unknowns? Picking a letter not entirely at random, just those beginning with “m” are mind-boggling: no-one knows about motoring, mobile roaming and medical care for British visitors to Europe.

As the government is fond of repeating, we’re “taking back control”. Yet whether you voted Leave, Remain or didn’t, I imagine you were not expecting such uncertainty about what the travel effects would be to prevail almost 40 months on from the EU referendum.

Before I left Seville, I called in on La Macarena, Our Lady of Hope, just inside the city walls. While the travel industry and I continue to hope for the best, they should be preparing for the worst.