The feature, contained in the NHS smartphone app, usually allows users to access a barcode or text records about which vaccine doses they have had.
Instead, users received error messages or a notice saying that high traffic volumes are "limiting access to the service".
Normal access to vaccine records was restored after more than three hours.
"The NHS Covid Pass service was temporarily unavailable between 11:45 and 15:15 today as a result of a technical issue with a global service provider that affected many different organisations," NHS Digital said in a statement.
Online, many travellers expressed concern that they might not be able to prove their vaccinated status at airports or other departure points without the app.