Brits travelling to Spain could face passport queue delays of up to five hours in the event of a no-deal Brexit, according to a consumer group.
Extra passport checks could add an extra 90 seconds per passenger at popular EU destinations, including hugely popular holiday destinations like Alicante and Tenerife.
Consumer group Which? Found that an average of 201 hours of additional immigration checks may be required every day at the airport if the UK withdraws from the EU without an agreement.
43 per cent of all passengers entering Alicante Airport are from the UK, according to the research.
Alicante is one of six Spanish airports in the top 10 for UK arrivals in Europe, with Tenerife South, Lanzarote, Malaga, Ibiza and Palma de Mallorca making up the other five.
Which? expressed concern that the Spanish government has yet to announce how it will tackle additional immigration checks.
It warned that officials may have to check UK holidaymakers' passport validity, passport expiry date, purpose and length of stay and whether they can support themselves financially.
The investigation calculated that it would take a single passport lane in an EU airport nearly five hours to process one flight full of UK passport holders.
Editor of magazine Which? Travel, Rory Boland, said: "Airports can be chaotic at the best of times, but if additional checks at passport control in Spain, Italy and other popular EU destinations are implemented in the event of a no-deal, it seems that very long queues are going to be an unwanted side effect.
"Until there is a deal or these airports announce simpler arrangements, you should consider what you may need if you have to fly to them - as it is very likely that you'll be in a queue for several hours.
"Make sure you have food, water and essentials for kids like nappies to hand."
