Lack of valid execution
If the will has not been made properly, as a result of failing to follow the correct procedure, this can lead to it being rendered invalid. For example, if witnesses were not present when the testator signed their will, it can be challenged.
Lack of testamentary capacity
With people living longer and dementia on the rise, there is more potential for a will to be challenged on the grounds that the testator lacked 'mental capacity' when writing their will. Lacking mental capacity is defined, under the Mental Capacity Act, as the inability for someone to make a decision for themselves due to an impairment of the brain. This can apply to dementia, brain injury and certain mental illnesses.
Lack of knowledge or approval, forgery and fraud
If a will has been drawn up by a third party, and the testator signs the will without full awareness or understanding of its contents, it can be deemed invalid. Similarly, if a will is forged or it has been affected by fraudulent statements, this will no longer constitute an enforceable legal document.
If the person making a will (ie the testator) was coerced to draft their will in a certain way or made changes to their will under duress, this can lead to it being rendered invalid. However, this type of claim can be difficult to prove.
Inheritance Act 1975
Even if there is no challenge of the validity of a will, it can still be contested under the Inheritance Act 1975. This allows dependents of the deceased (including cohabitees) to make a claim against the estate if they can prove that they were financially dependent on the deceased and were not sufficiently provided for in the will.