In an exclusive interview, the service’s chief executive, Bernard Herden, said: “The Sunday Times has drawn our attention to certain issues and we will address them. There’s going to be clearer guidance and we’re looking to make things easier for people with children.”

Stringent laws governing passport photos were introduced in September as part of the government’s undertaking to meet international biometric standards. Pictures need to be machine-readable, and tilted heads, unruly hair and even smiles are prohibited. The requirement for expressionless faces has caused particular frustration for parents attempting to apply the new rules to children, and the UKPS says it will issue new guidelines within the next 10 days.

Though he blamed photo-booth manufacturers for failing to adapt machines in time to meet the new standards, Herden also accepted that technical issues, inconsistent standards and spurious rejections within UKPS offices were causing problems, with 1 in 10 photographs rejected.

“Ten per cent is not a tolerable situation,” he said, “and if there’s anything we can do to improve that, we will.”

Asked why the UKPS was rejecting photographs that had been approved by the Post Office’s £7 Check & Send service, Herden said it was difficult for Post Office staff to know what was acceptable for UKPS technology. “We need to get more precise with them. It’s a learning curve, and we have some joining up to do.”

The Post Office has promised to pay £71 for Fast Track processing for clients whose travel plans are jeopardised by its mistakes.

“The trust that customers place in us to ensure that their passport applications are processed in time is extremely important to us,” it said.

Times recommends

• The 10 best bits of travel gear
• Soho's classy, cheap new townhouse
• Germany's perfect Christmas city