Passports usually have two numbers
1. The Passport book number (in red colour on the first page - normally starts with letters)
2. The passport travel document number (the one that normally appears on the same page where the picture is)
When reading documents for other countries I noticed that they specify the difference between the two, while in UK applications they only ask for 'passport number' and I have never felt sure about which one to use when filling an application and I guess it has to coincide with what is stated in the CoS by the employer (?).
Thanks for the insights
