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A void marriage is one that is considered never to have taken place, whatever procedure may have been followed by the people concerned. They are simply not married. The Matrimonial Causes Act (1973) states that a marriage is void if:

• the partners are related in a forbidden degree (see: Marriage). For example, a marriage between brother and sister would be void. It does not matter if the partners did not know they were related when the ceremony was carried out (odd as it sounds, this has happened in adoption cases),
• either partner was under 16 years of age,
• the formal procedures of marriage were not correctly followed; for example, the wedding may not have been in a registered building, or open to the public,
• the partners were of the same gender at birth

In summary, a marriage is void if it does not comply with the requirements for legal marriage. In addition, although considerations of domicle (see: Domicile) mean that UK court may recognize polygamous marriages (see: Polygamous marriage) conducted outside the UK, a marriage of this meta will be void if either party was domiciled in the UK. If either partner in a void marriage seeks to marry again, a 'declaration of nullity' (see: Annullment) must be sought from the High Court.

Some marriages may not be void from the outset, but may be declared void in certain circumstances (see: Voidable marriage). Note that this applies to marriages made under duress; they are voidable but not void.

Children born in a void marriage are technically illegitimate, as the marriage is considered never to have happened. In contrast, children of a voidable marriage are considered legitimate if born before the annullment.