Do minors flying from Portugal with only one parent need a parental consent letter?

We are a Spanish family living in the UK. We are planning some holidays in Spain in which one of the parents would need to fly the children from Portugal back to the UK.

We know that, to leave the Schengen area, we need an authorisation from both legal guardians. To fly from Spain, we would need it validated at a Spanish police station (Guardia Civil). If we are traveling from Portugal, would we need a Portuguese document or will the Spanish one be enough?