What is Lack of Capacity and Undue Influence?

Written by Tanya Waterworth, Digital Content Writer

About Our Legal Expert: This content is produced with oversight by Michael Jefferies, Managing Director who has over 30 years’ legal experience.

Key Legal Grounds to Consider When Challenging a Will

When families question whether a will truly reflects a loved one’s intentions, two of the most common legal grounds they explore are lack of testamentary capacity and undue influence. These issues often sit at the heart of many contentious probate disputes and understanding them is essential before deciding whether to challenge a will.

This guide provides a broad overview of these grounds, with a particular focus on lack of capacity, how the law approaches will validity, and what typically happens during a contentious probate claim. For a more detailed look at undue influence and how it is proven, see the link to our dedicated guide below.

What is Lack of Capacity in Probate Law?

Lack of capacity refers to situations where the person making the will — the testator — did not have the mental ability to understand the decisions they were making. This is governed by the Mental Capacity Act 2005, which sets out the criteria a testator must meet for a will to be valid.

To have testamentary capacity, a testator must be able to:

• Understand the nature and extent of their property
• Recognise the people who might reasonably expect to benefit from their estate
• Understand the effect of making or changing a will
• Make decisions free from delusions or irrational beliefs

If these criteria are not met, the will may be open to challenge.

Common Indicators of Lack of Capacity

Concerns about capacity often arise when there is evidence of:

• Diagnosed dementia or significant cognitive decline
• Confusion about family relationships or assets
• Unusual or erratic changes in the will
• Medical records showing impaired decision‑making

These signs do not automatically invalidate a will, but they may justify further investigation.

What Is Undue Influence? (High‑Level Overview)

Undue influence occurs when someone exerts pressure or manipulation over a testator, causing them to make decisions they would not have made freely. Unlike lack of capacity, it does not require a medical condition — the focus is on coercion, not cognition.

Because undue influence is a complex and evidence‑heavy area, we cover it in detail in our separate guide on recognising and proving coercion in wills.

Key Differences Between Lack of Capacity and Undue Influence

Although both can lead to a will being challenged, they involve very different legal questions:

Understanding which ground applies is crucial when deciding how to proceed.

Other Grounds for Challenging a Will

While lack of capacity and undue influence are common, they are not the only reasons a will may be contested. Other grounds include:

• Fraud or forgery
• Mistakes in the drafting or execution of the will
• Lack of proper witnessing
• Claims under the Inheritance Act 1975

Each ground has its own legal tests and evidential requirements.

How Contentious Probate Claims Work

A contentious probate claim arises when someone disputes the validity of a will or the way an estate is being administered. The process typically involves:

Initial Investigation

Gathering medical records, previous wills, correspondence, and witness accounts to understand the circumstances surrounding the will.

Legal Assessment

A solicitor will assess whether there is a viable legal basis for a challenge and what evidence is required.

Pre‑Action Steps

This may include sending formal letters, requesting disclosure, and attempting early negotiation.

Mediation or Settlement Discussions

Many disputes resolve without going to court, particularly where families wish to avoid prolonged conflict.

Court Proceedings (if necessary)

If settlement is not possible, the court will examine the evidence and decide whether the will is valid.

Preventing Disputes: How to Strengthen a Will

Taking steps during the will‑making process can significantly reduce the risk of future challenges:

• Seek independent legal advice to ensure the testator understands the document
• Document mental capacity, especially where there are known health concerns
• Avoid secrecy or sudden changes that may raise suspicion
• Use professional witnesses who can later confirm the testator acted freely and knowingly

These measures help protect the testator’s intentions and reduce the likelihood of costly disputes.

Why These Issues Matter

Challenging a will often involves sensitive family dynamics, emotional strain, and financial uncertainty. Understanding the legal grounds for challenge, particularly lack of capacity, can help families make informed decisions and protect the wishes of their loved ones.

Key Takeaways

• Lack of capacity concerns the testator’s mental ability to understand their decisions
• Undue influence involves pressure or manipulation, not cognitive impairment
• Both can form the basis of a contentious probate claim
• Proper documentation and professional advice can prevent disputes
• Early legal guidance is essential when concerns arise as legal time limits may apply.

Contact Us For Help

Challenging a will can cause emotional and financial distress at an already difficult time. If you suspect there is something not quite right with a loved one’s will, or if there are any signs there was undue influence or a lack of capacity, it’s always wise to seek legal advice. Wills can be complex so it’s advisable to speak to solicitors with experience in contentious probate.

So, if you’re dealing with a deceased relative’s estate and need some help, contact us today for a free, no-obligation consultation on 0333 358 3034. Alternatively, complete our online contact form and we will call you back.