TAP airline

mneslcl, your posts are simply misleading. Any EU citizen has a right to travel everywhere in the EU with just his or her national ID card, no need for passport. This applies regardless of the country in question being in the Schengen area or not.
The Schengen area is about border controls, not passports. A EU citizen travelling between the Schengen area and a Member State not belonging to the Schengen area (e.g. France to the UK or Romania to Portugal) will be subject to immigration check at the border but he or she is legally entitled to cross the border with just his national ID card.

I'm with meczko on this one. I frequently travel from Schengen to the UK with just my Belgian id card, no problem. Maybe I'll give TAP a call tomorrow, I'm kind of intrigued.

mnescl it is you who needs to read it more carefully it clearly state that those countries that are not Schengen you require a valid passport or id card. For travelling between Schengen countries you technically do not require a passport or id card.

Richiavo, it is not true either. Technically, you still need to hold valid proof of identification (either passport or ID card) when travelling across a state border even inside the Schengen area. The difference is that inside the Schengen area it is not checked at the border but spot checks inside Member States are possible.
To sum it up - a national ID card of a EU Member State or a passport are required to cross borders between EU Member States, with ID card being perfectly enough. This applies regardless whether you travel inside or outside the Schengen area. The only difference is that there are no immigration checks at the actual border inside the Schengen area, while those checks are still done on the border of Member States which do not belong to Schengen area (UK, Ireland, Romania, Bulgaria, Cyprus, Croatia).

The rules at the borders aren't followed strictly, as I experienced. I used to fly often from Sweden to the UK and Portugal. For flights to the UK I had to get through Swedish passport control and show my passport, before departure. The gates to non-Schengen countries are in a separate area of the airport.
On arrival in the UK (country outside Schengen), I showed either my Swedish ID or driving license, and I was getting through without problems. Afaik, driving license shouldn't be accepted as an ID at border check, but it was. Similarly in Portugal, although that is much easier when flying between two Schengen countries, where there's no border check.

Fieldgate, even if Swedish immigration officers ask for passport, you can still insist on flying to the UK only with your Swedish ID. That's your legal right and Swedish immigration officers are certainly qualified enough to know it. Obviously sometimes it is easier just to show the passport than to explain the thing.
A driving licence is formally not the right identification for cross-border travel within the EU but there is apparently also some discretion for UK immigration officers. If on seeing you driving licence they are reasonably convinced that you are an EU national, why not let you in?

By the way, in Portugal on many occasions I was asked for passport or had to fill in questionnaires with a "passport number" field. On hotel check-in, opening a banking account, getting a tax identification number etc. It was always perfectly acceptable to show my Polish ID instead of passport. Sometimes I needed only to explain its content, as there are not so many Polish speakers in Portugal :)
It seems that asking for "passport" is in many cases just a left-over from pre-Schengen times. Anyway, it is already generally known that a EU citizen from another member state does not need to have it.

You should have just entered your ID card number where it says passport. When asked for it at the airport, show your ID with the number.

This is Portugal, nobody is going to make a special "ID card number" field for you.