For civil partners
If you and your partner register your partnership and then one of you dies, the surviving partner inherits in accordance with the rules of intestacy in the same way as spouses if there is no Will. They have the right to register the death of their partner and are eligible for bereavement benefits. They have the right to claim a survivor pension, have tenancy succession rights and can claim compensation for fatal accidents or criminal injuries. They are also recognised under inheritance tax rules.
As with marriage, you still need to make a will after registering your partnership as any wills made beforehand become void. You will then be recognised under the intestacy rules, and will benefit from the same exemption from inheritance tax as married couples. If you have existing wills you can either arrange for them to be amended before you register so they allow for the eventuality of a civil partnership, or you can simply have them amended afterwards.
If you choose not to make a Will, your civil partner will inherit under the rules governing intestate estates, which will take into account children from existing or previous relationships.
For couples not in civil partnerships
If a same-sex partner dies without leaving a will (intestate) the remaining partner could lose any claim on his or her partner's estate. The laws of intestacy do not recognise 'unpartnered' survivors. So if your partner dies intestate their estate will go to your partner's parents or, if they are not living, their siblings; if there are no siblings, then remoter relatives.
It is clearly very important for lesbian or gay partners to make wills.
If there is no will or no provision has been made for a remaining partner the only other avenue is to make a claim under the Inheritance (Provision for Family and Dependants) Act 1975.
To make a claim it is necessary to show that you have been 'maintained, either wholly or partly by the deceased'. In fact the scope of this provision is quite limited. Where there has been an equal relationship in which both partners have contributed to their life together, it may not be possible to show dependence.