You generally cannot stay in the UK for any amount of time without a valid passport, as of February 25, 2026. Following the full rollout of the Electronic Travel Authorisation (ETA) system, almost all visitors (except British and Irish citizens) must present a valid passport and a pre-approved ETA to enter the country. EU, EEA, and Swiss citizens, who previously could enter using just a national ID card, are now required to use a digital passport for all border crossings. For those with valid immigration status or a visa, the stay duration is typically capped at six months for standard visits. If you are a dual British citizen without a valid UK passport, you may face significant delays and identity checks at the border, as nationality must be verified before entry is granted, effectively ending the era of "passport-free" travel for most.