Adultery in the UK: How is it Defined?

Adultery is a highly emotive subject. It has meant the end of many marriages and caused huge amounts of heartbreak.

Although many may understand adultery as an affair outside of a marriage, UK law is quite specific in its application. This means that not everyone who has discovered an infidelity is able to use adultery to legally end a relationship.

We’ve put together this guide to help you understand how the law defines adultery in the UK.

What is adultery?

Adultery is defined in UK law as a husband or wife having sexual intercourse with someone else of the opposite sex. This means that even if you find that your partner has kissed someone else, messaged someone else, been using dating sites or having a relationship that doesn’t include sex, it will not be considered adultery in a UK court. Not all physical intimacy will be seen as adultery.

It is used as one of five reasons for divorce. These reasons all have to fall under one ground for divorce: the irretrievable breakdown of a marriage.

The four other reasons are:

  • unreasonable behaviour
  • desertion
  • separation of two years with the consent of both parties to divorce
  • separation of five years without the consent of one partner.

The UK is set to introduce no-fault divorces, however, in an attempt to reduce conflict during the process.

It is not possible to use adultery as a reason for divorce if two people have lived together as a couple for more than six months after the infidelity was discovered. This means that if you want to cite adultery as a reason for divorce, you will have to make your mind up about ending your marriage as soon as you can. This can be a particularly difficult time so it is a good idea to seek the support of those around you.

Can same-sex couples commit adultery?

With the introduction of same-sex marriage, you might have thought the law around adultery would have been updated to reflect this change. However, adultery is still defined as sex between a married woman and a man other than her husband or a married man and a woman other than his wife.

This means that if an affair outside a marriage was between two people of the same sex, it will not be possible to claim adultery as a reason for divorce. However, a person in a same-sex marriage may commit adultery if they have an affair with someone of the opposite sex.

Civil partnerships cannot be ended due to adultery. This is just one of the differences between civil partnership and marriage. The reasons for ending a civil partnership include unreasonable behaviour, which could include infidelity. They also include desertion, separation for two years with the consent of both parties and separation for five years without consent.

The Matrimonial Causes Act, the law relating to divorce, was introduced in 1973. We have since seen the introduction of same-sex civil partnerships and marriage. It remains to be seen whether the UK government will change its divorce laws.

How can I prove adultery?

If you have discovered that your husband or wife has been unfaithful and you want to use adultery as a reason for divorce, you will have to prove it to the court. This can be done by your partner admitting their infidelity. You could also provide evidence. Common forms of proof include photos, video and text messages confirming what happened.

However, if they don’t want to admit it, you will find it difficult to prove that adultery has taken place. This could mean that a court will not find that your marriage has irretrievably broken down.

You may find that it is easier and less acrimonious to use one of the other reasons for divorce if you can’t prove adultery and your spouse is not willing to admit it. You may choose to use unreasonable behaviour as your reason for divorce, explaining that your partner has behaved in a way that you cannot be expected to live with. Having an extra-marital relationship can be seen as exactly that.

Naming the person your partner was unfaithful with is not commonly done. This means it is typically not used as a method of proving adultery. Naming the other person – known as the co-respondent in this situation – is usually avoided in order to keep the divorce process as amicable as possible. In fact, UK courts have directed that in all but exceptional circumstances, the co-respondent should not be named.

What to do

Due to the complexities around adultery in UK law, the best thing to do is get help from an experienced divorce lawyer. They’ll be able to help you work out what most appropriate next steps for you are.

If you think you’re ready to end your marriage, just get in touch with First4Lawyers. Our compassionate and understanding advisors can talk you through your options, helping you to come to terms with your divorce.

Just give us a call, request a call back or make an enquiry online.


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