I wondered where this information was being requested, and in the on-line application form for a British passport I found the following instruction:8.2 Being in possession of an existing passport or the ability to obtain a passport from another country is part of the decision making process in considering an application for a British passport. It is an important element of the identity confirmation process.
8.3 Failure to disclose this information during the application process is a criminal offence and may render the applicant subject to criminal prosecution. HMPO will consider whether the matter should be referred for criminal investigation and whether the existing passport should be withdrawn. A withdrawal of the passport would be on the grounds of:
Are they seriously now saying that dual nationals who have only ever held British passports must now attempt to obtain a passport of another nationality? There's no "if any" qualification to the instruction.Dual nationals:
Applying from the UK - you must send us the original non-British passport and provide the number below.
At least those non-Irish nationals entitled to Irish nationality aren't obliged obtain the nationality by applying for an Irish passport (some British citizens could lose their jobs for becoming Irish), but it seems that they have to confess that entitlement, though I haven't found exactly where the confession is wrung out of them. (There may be rare cases where such an entitlement, arguably deservedly, puts one's acquired British citizenship at risk.)