Does anyone know how to officially calculate the pound to euro exchange rate when declaring UK income (interest/pension, etc) on the Spanish tax return? I’ve read in some places that it should be based on the Central European Bank rate on the exact day payment is received rather than using an annual average, but I can’t find any official source, i.e., AEAT.

Go to the BOE search page and enter the following:

• Tick only Sección III, untick the rest
• Título: cambios del euro
• Departamento: Banco de España
• Fecha de publicación de: the start and end dates

The date of this exchange rate is e.g. "correspondientes al día 23 de abril de 2026" in the title at the top. It's not published on weekends or public holidays though.

I calculate the rate for each day that I need to because I haven't been able to find anything official which says it's okay to take the average for the year or use the exchange rate as it was on the last day of the financial year.

Thanks for the info. So you use banco de España rate, which i think is based on central european bank anyway. And, yes, most of what i read suggests you should use the rate on the specific day, not annual averages. The difference is not generally much but i would prefer to follow the rules, the problem is finding out what they are!

To be 100% correct you should use the ECB rate on the day you received the money which is what I've been dong for a number of years without issue.
Although the ECB annual figure is commonly bandied about I have yet to see an average published either by AEAT or anyone equivalently official in Spain.

The bottom line is that you need to be able justify the figures you submit if challenged and calculating as per above is the only method will always be completely correct and beyond question.

Yes, I think that's the way to go.