When you marry, any existing will is automatically revoked (cancelled), unless it contains a clause in anticipation of marriage, leaving it invalid. This means that you will not have a valid will and if you do not make a new one, your estate will be dealt with under the rules of intestacy. Depending on the size of your estate and your circumstances, your estate may be split between your spouse, and any children you have or your siblings or even parents.
Unlike a marriage, divorce does not automatically revoke your whole will. Instead, if you divorce or end a civil partnership, your will is treated as if your ex-spouse or civil partner had died on the date that the decree absolute or the dissolution of the partnership was issued.
By way of example, if your ex-spouse or civil partner was named as your executor, that appointment will be ignored and your reserve executors will take their place. It also means that any gift that you made to them in your will does not take effect.
If you still want to leave something to your former spouse or civil partner, you need to make this clear, either by way of a new Will or perhaps by means of a codicil (an instruction added to a will) stating that whilst you are no longer married or in a civil partnership, they should still receive the provision you made for them in your will.
Rules are different where you and your former spouse had made mutual wills, containing an express agreement not to revoke or change the terms without the consent of the other. Whether these wills are still binding on divorced couple depends on the wording of the will in question and is assessed on a case by case basis.
Whether you are marrying, separating or entering into a long term relationship, it is always worth considering whether you should amend your will. There are many well publicised examples of dependents left struggling or fighting in court because of lack of a will, or lack of clarity in it.