A Unilateral Notice is an entry on the Land Register (applied for using Land Registry Form UN1) in England and Wales. It formally records a third party's claimed interest in a registered property (e.g., an option to purchase), and crucially, can be entered without the property owner's immediate consent. Its purpose is to provide public warning of the claimed interest, affecting anyone dealing with the property. While it doesn't prevent a sale, it can complicate it, and the property owner can challenge it, requiring the claimant to prove its validity.