From British Embassy in Madrid.

Official information regarding entry into Spain

Travel Update

Today the Spanish Government has announced that the current travel restrictions will be extended until 19 January 2021 (see BoE bulletin) for further details. 
Entry to Spain is restricted to Spanish nationals and those who are already legally resident in Spain. We have updated our travel advice accordingly. https://www.gov.uk/foreign-travel-advice/spain

If you are resident in Spain, you should carry your residence document (the green paper EU residence certificate or the new TIE), as well as your valid passport when you travel. The Spanish Ministry of Foreign Affairs has confirmed that whilst these restrictions are in place only those residents who can prove their status as residents through the green paper EU residence certificate or the TIE card may enter Spain.

This applies to passengers arriving in Spain from the UK by air and sea only (not overland travel). The restrictions also include flights from the UK to the Balearic Islands and Canary Islands. If you were due to travel to Spain please contact your travel operator before departure.