Tens of thousands of photos in passport applications have been rejected for failing to meet new rules introduced in September last year to combat fraud.
The UK is one of 40 countries preparing biometric passports
UK Passport Service (UKPS) figures showed that of 597,863 applications in the eight weeks from 12 September some 81,927 photos - 13.7% - were rejected.
The most common problems were obscured eyes, an unsuitable facial expression and incorrect paper, the UKPS said.
A spokesman said rejections were now below 10% and would continue to fall.
UKPS had expected 20-25% of photos to be rejected immediately after the rules came in.
"We thought there would be far more rejections initially but the public is getting used to the new rules," the spokesman said.
Introduced on 12 September, these demand:
- The photo must be 45mm by 35mm and printed on normal photographic paper
- The photo is taken against an off-white, cream or light grey background
- Subjects look straight ahead with mouths shut and "neutral" expressions
- Subjects are on their own in the picture and their head and shoulders take up 65% to 75% of the frame
New biometric passports have the picture saved on a chip, which can be scanned to reveal the holder's facial image.
The UKPS spokesman said: "We admit that people have been inconvenienced - that's the downside of any new measure, especially enhanced security measures."
But he added: "Facial recognition technology demands certain standards.
"There is only so much that we can play with the rules - the biometric needs to be good enough that other countries' border controls are going to be able to recognise it."
UKPS found after eight weeks of the new rules that the main reasons for rejections were "eyes not clearly visible", an "unsuitable pose" such as eyes not looking at the camera or an open mouth, and incorrect paper or the submission of school photographs that cannot be used for copyright reasons.
The service subsequently issued new guidance to applicants, explaining the rules.
It has also agreed with other countries to relax the rules for photographs of under-fives.
"There is an unacceptable level of passport fraud," the spokesman said.
"This is going to deliver a step change in the level of passport security."
Holders of existing passports do not need to have them updated but will have to comply with the new rules when they apply for replacements.