Most people use adultery and infidelity interchangeably, but in UK divorce law they have always meant different things. That distinction has become less significant for the divorce process itself, but it still matters in financial proceedings and in cases involving children.
Understanding where the line sits, what changed under no-fault divorce, and where PI evidence still has a role helps anyone navigating separation make better-informed decisions about what they actually need.
What is the legal definition of adultery in UK law?
Legally, adultery has a narrow definition. Under the Matrimonial Causes Act 1973, it was defined as voluntary sexual intercourse between a married person and someone of the opposite sex who was not their spouse. That definition excluded same-sex affairs, emotional relationships, and other forms of intimacy that most people would consider infidelity.
Infidelity is a broader, everyday term covering any behaviour that constitutes a betrayal of the relationship, whether sexual or emotional, same-sex or otherwise. Before the 2022 reforms, a spouse who had an emotional affair or a same-sex relationship could not be accused of adultery in the legal sense, though their behaviour might have been cited as unreasonable behaviour instead.
How the no-fault divorce law changed things in 2022
The Divorce, Dissolution and Separation Act 2020 came into force on 6 April 2022 and fundamentally changed how divorce works in England and Wales. Under the previous system, a spouse seeking divorce had to rely on one of five facts, including adultery, to demonstrate that the marriage had irretrievably broken down. That required making accusations and, in the case of adultery, producing evidence.
No-fault divorce removed that requirement entirely. Either or both spouses can now apply for divorce by making a statement that the marriage has irretrievably broken down. No reason needs to be given, no conduct needs to be proved, and the other party cannot contest the divorce on the basis that they dispute the reason. The court does not investigate blame.
The practical effect of no-fault divorce on adultery is significant. Proving adultery in divorce proceedings in the UK is no longer necessary to obtain a divorce. The distinction between adultery and infidelity, which once had real procedural consequences, now has little bearing on the divorce application itself.
Does proving infidelity still matter in UK divorce?
For the divorce application, infidelity no longer carries any weight. However, divorce and financial settlement are separate processes, and the picture is a lot more nuanced in financial remedy proceedings.
Family courts in England and Wales take a needs-based approach to financial settlements. The starting point is a fair division of assets, and conduct is only taken into account if it would be inequitable to disregard it. In practice, that is a high bar. Infidelity alone rarely affects how assets are divided, and courts are reluctant to use financial settlements as a mechanism for punishing a cheating spouse.
That said, infidelity evidence can become relevant in specific circumstances. If a partner has spent significant marital assets on an affair by paying for accommodation, travel, or gifts for a third party, that financial conduct can be raised in court.
Whether adultery matters in a UK divorce settlement depends on the facts of the individual case rather than the fact of infidelity itself.
Using PI evidence in financial settlements and custody
PI evidence does not become irrelevant just because no-fault divorce has reduced the procedural significance of adultery. Under UK infidelity divorce law as it stands, evidence of a partner’s lifestyle, living arrangements, or financial behaviour still matters in financial remedy proceedings. In some cases, it matters to child arrangements too.
If a former spouse claims they cannot support themselves while living with a new partner, evidence of that cohabitation can directly affect maintenance payments. And if there are concerns about a child’s welfare around a new relationship, documented evidence of that relationship can be relevant to a court and may affect contact arrangements.
High-value cases work the same way. If a partner spent marital assets during an affair, on accommodation, travel, or gifts, that can be raised as misconduct in court. So, ultimately, the value of the evidence has moved. It now sits with the financial and welfare questions that follow the divorce rather than the divorce application itself. That is a meaningful distinction for anyone weighing up whether an investigation is worth it.
When an infidelity investigation still makes sense
The legal framework has changed, but the reasons people commission infidelity investigations are largely the same. Confirming a partner’s behaviour, establishing whether they are cohabiting with someone new, documenting financial conduct that may not have been disclosed, and gathering information relevant to a child’s welfare are all things a professional investigation can still address.
What has changed is where that evidence lands. It is less likely to affect the divorce itself and more likely to matter in the financial remedy or child arrangements proceedings that follow. A professionally compiled report produced through lawful methods is admissible and defensible.
Book an infidelity investigation with B25
B25 advises clients on what an investigation can realistically achieve before any work begins. For anyone navigating separation and uncertain whether it is worth pursuing, that initial conversation costs nothing and commits to nothing.
The team brings a combined background across military, intelligence, and policing, with accreditation from the ABI, IPI, and GBC. Every case is handled with complete confidentiality from first enquiry to final report.
Considering an infidelity investigation for divorce proceedings? B25 can advise on what is achievable and how. Call us on 0800 593 2525 or use our online form to get the answers you need.