There is no single person who is automatically legally responsible for an adult with dementia in the UK. Adults retain legal autonomy unless they are formally assessed as lacking capacity to make a specific decision under the Mental Capacity Act 2005. If a person has lost capacity, a Lasting Power of Attorney registered before they lost capacity gives the named attorney legal authority to make decisions on their behalf. If no Lasting Power of Attorney is in place, adult children and relatives have no automatic legal authority, and it may be necessary to apply to the Court of Protection for a deputyship order. Setting up a Lasting Power of Attorney early, while the person still has capacity, is strongly advisable.