The HM Passport Office has warned that Brits nationwide could unknowingly possess an invalid travel document. Valid passports are essential for international travel, with the document typically lasting a decade - or five years for a child.

At present, a UK passport costs £94.50 through an online application, or £107 via a paper form, whilst a child's passport is £61.50 when applying online, or £74 using the paper form. Upon receiving your passport, there's something you must do to validate it - otherwise, you'll be unable to use it, reports the Daily Record.

In a recent alert on X, formerly Twitter, the Passport Office warned: "Don't forget to sign your passport! Most people over the age of 11 will need to sign their passport, and can't use it until it's signed.

"Unless your passport states 'The holder is not required to sign', you must use a black ballpoint pen [and] sign on the holder's signature line."

Additional guidance on Gov.uk states: "A sign after receipt passport is not a valid travel document until the holder has signed it. HM Passport Office advises customers to sign their new passport as soon as they receive it.

"A passport may not be valid for travel if a customer has made a mistake when signing (for example, they have signed the wrong name). Passports that were issued with a digitally printed signature remain valid until their expiry date."

You should never sign in pencil or coloured ink, include any additional information, or use a signature that differs from your usual one - as these actions could invalidate your passport. It's also important not to erase or cross out a signature, as doing so could damage the page, or apply correction fluid on it.