If you have dual nationality, the way you apply for a UK ETA may depend on the nationalities you hold.
If you are a dual national with British or Irish citizenship, you cannot apply for a UK ETA.
In this case, you should travel to the UK using a valid British passport, a valid Irish passport, or another valid passport containing a certificate of entitlement.
If you do not have British or Irish citizenship, you may apply using a passport that is eligible for a UK ETA.
When applying, use the passport you will travel with, because your approved UK ETA will be linked to that passport.
If you apply with one passport but travel with another, you may have problems during boarding or entry procedures.
Before traveling, carefully check your nationality, passport type, and UK ETA eligibility.