Help - how to see which is additional British passport

[Jantlet]

DH has an additional passport issued for frequent overseas business travel. Both this, and his original passport were issued on same date. Both are now expiring.

He needs to apply for a replacement of the original - but how can I tell which is the original and which was the additional?

Are the passport numbers similar? I'd assume it was the earliest number.

Not the point, I know, but I didn't even realise you could do this. 😁

As a per a pp, I might assume the earliest passport number? Don't you need to renew both of them now though, to stay within the rules when travelling of having minimum validity periods left on the date of your return?

Does the original one include any particular observations?

Is he planning to request a renewal of the additional one?