Applying for a first UK passport with two non-British passports

[uk-ua-pair]

I was going to apply for my first UK passport online, but I found that I must send "any valid passport that hasn't expired" via post? I have two non-British passports, do I really need to send them both? I checked requirements if I would be applying via post and it's only passport + naturalisation certificate, they don't mention to send all other passports...

[alterhase58]

Are they both unexpired?
If you are using both of them I suggest submit both - HMPO want to ensure that the identies/names are the same in all valid passports.

[uk-ua-pair]

Yeah... I called the help line and they said the same but also advised to attach a letter to ask to return the documents asap because of expected travel soon, so they should be returned maybe quicker...