The UK Passport Service is suffering high rejection rates with its biometric facial recognition technology, it has admitted.

The agency said recently revised guidelines for passport applicants have reduced the problem, but it is still rejecting one photo in every ten. In December, it was one in seven.

Photos are now scanned to generate a biometric based on key facial features, which is then stored in a chip on the passport. The biometric technology needs a full-face pose, a "neutral expression", and specific features such as the eyes and closed mouth clearly visible.

A UKPS spokesman said facial recognition is already being used to detect fraudulent or multiple applications, but that the ultimate plan is to be able to compare the stored biometric with a live picture taken at a border crossing. "To do that, the photo has to be of sufficient quality," he said.

He added that as well as difficulties with the pose, problems have been encountered with watermarked photo paper confusing the scanning software, and with digital photos printed at home. "We blow up the picture a lot, and a home printer is unlikely to be high enough resolution," he said.

UKPS, which expects to issue seven million passports this year - more than any other country except the US - denied that its biometric technology was at fault.

"We are improving the rejection rate - it's a submission issue, not technology," its spokesman said. He noted that the new passports are in response to US demands and follow standards set by the International Civil Aviation Organisation (ICAO).

However, privacy campaigners have highlighted the unreliability of facial recognition in many trials, with error rates up to 50 percent in one, while unpublished studies suggest that the technology's sensitivity to lighting conditions can give false results in as many as one in 10 scans.

Indeed, the US National Institute of Standards and Technology has said that facial recognition is less accurate than fingerprinting. However, it is popular because of its relative simplicity of implementation - and of course its potential for covert use.