Mismatched passport details
The single most common cause — a name, number, or date that doesn't match your passport exactly.
Incomplete or inconsistent information
Gaps in travel history or contact details that don't line up with your application can trigger a manual review, and sometimes a rejection.
Previous immigration issues
A prior visa refusal, overstay, or deportation from the UK (or elsewhere) can affect the outcome and may need a full visa instead of an ETA.
Poor quality photo
Photos that don't meet the requirements (blurry, wrong background, glasses glare) can delay or block approval.