Travel to Spain with two passports

[DosPasaportes]

I have a UK passport, and a Spanish passport. Spain doesn’t recognise dual nationality so I had to declare that I was renouncing my UK citizenship when I applied for my Spanish passport. However I was advised off the record by a Spanish consulate employee that in practice many people keep both without any problems. I’m flying to Spain soon and gave my UK passport details to the airline when I checked in, but I’d like to use my Spanish passport at immigration when entering and leaving Spain. Do the immigration authorities cross check their records with the airline and will they detect that I’ve used my UK passport to check in and board the flight?

Why didn't you just use your Spanish passport details?

Don't you have to add them to the booking?

Yes they can check. Assuming you aren't actually on the plane yet or the flight has closed the airline can change your details to use the Spanish one. Are you flying today?

I'm not flying today. When I booked the flights I entered my normal surname out of habit and not the two surnames which appear on my Spanish passport. At check-in I decided that it would be more straightforward to enter my UK passport number. I can't see an easy way to change this on the airline's booking system now, so I think it's safest to use my UK passport all the way through. I'd been hoping to avoid long queues at immigration, but I don't think it's worth the stress.