The Mental Health Act 1983 defines a list of certain people who can be treated as the 'nearest relative' of a patient.
A 'nearest relative' has a number of important powers and functions, including the right to discharge a patient who has been formally detained in hospital, make an application for a person to be admitted for assessment, treatment or guardianship and also to object to applications for treatment or guardianship being made by a social worker.
Only certain categories of people can become a 'nearest relative'. First in the list are civil partners and spouses, then unmarrried heterosexual couples who have lived together for six months.
Previously, a same-sex partner could only become the nearest relative if they had lived together for five years.
The introduction of the Civil Partnership Act means that civil partners are automatically entitled to the same next of kin rights as married couples. The term 'next of kin' is widely used, but there is no statutory definition. In practice the general rule has been to recognise spouses and blood relatives as next of kin - this now extends to civil partners.
Although the term is vague, hospitals are frequently loath to give information to anyone other than those they regard as next of kin.
Given the development of case law in other areas we can see nothing that need prevent a same-sex partner describing his or herself as next of kin, although this may not always be accepted.
In relation to medical treatment, outside the mental health field an adult is the only person who can give valid consent to the administration of health care.
In cases where a patient is unable to give consent themselves healthcare professionals are obliged to treat a patient in accordance with his or her best interests. In reality they would undoubtedly discuss the position with partners or relatives, so establishing that you should be treated as next of kin is important.
In a worse-case scenario a same-sex partner not in a civil partnership could be excluded from all major decisions about treatment and we have been notified of instances where this has happened.