Terminal 1 - after checking in we were given boarding passes for Gate D. We then proceeded through the security search and saw that gates A B and C were down the escalator and that gate D was to our left and gate E to the right. We could see passports were being checked at D so we proceeded forward. Wrong. Don't go through passport control until you are ready to board your plane. Once through the security check go straight down the escalator to the shops and restaurants. The big glass balcony that houses the security check area and Gates D and E give you the impression that once you have passed through passport control you can still proceed downstairs to the shops and restaurants - you can't. Once we realised our mistake I felt such a fool that I thought I would not be telling anyone until I realised that the balcony had loads of people wandering around wondering how they could now get down to the shopping area. Although the passport booth had a sign saying you cannot return back once having passed through this just conveyed the message to most people that you could not leave the airport. We did not realise it meant you would no longer have access to the shopping/restaurant area! Note to the Barcelona airport authorities: Please get the signage at the check in desks and through security to read "Don't go through passport control until you are ready to board your plane." You are losing revenue with all those people stuck up on the balcony with boarding passes for gates D and E!