An eTA is electronically linked to a traveller's passport. It is valid for up to five years or until the passport expires, whichever comes first.
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People from the UK, don't require a visa to travel to Canada. However, just like all the other visa-exempt nationalities, except for the nationals of the US, as a UK citizen, you must apply for a Canada ETA before departure. That is, of course, if you plan to travel by air.