Mutual Wills Doctrine UK: Dufour v Pereira, Floating Trusts and Mirror Will Risks
Updated 21 May 2026 · 9 min read · Wills & Trusts
When two people make wills on identical terms and agree never to change them, those wills may become mutual wills — creating a binding legal obligation enforced by a constructive trust in equity. The doctrine traces back to Dufour v Pereira (1769) and remains live law, though modern practitioners almost universally advise against it.
Mirror Wills vs Mutual Wills: A Critical Distinction
Mirror wills are two separate, independent wills in identical (or near-identical) terms — each spouse leaves everything to the other, remainder to the same children or beneficiaries. Mirror wills are very common and entirely revocable: either party may update or revoke their will at any time, without consent or notice.
Mutual wills are mirror wills plus a binding agreement not to revoke. The agreement to keep the wills unchanged is enforced in equity: once the first testator dies and the survivor takes the benefit, a constructive trust is imposed over the survivor’s estate to protect the agreed beneficiaries.
The mere fact that two wills are in mirror form does not make them mutual wills — there must be a proven agreement. Many couples unwittingly believe their mirror wills are “locked in” when they are not, and others make mutual wills without appreciating the consequences.
The Rule in Dufour v Pereira
In Dufour v Pereira (1769) 1 Dick 419, Lord Camden held that execution of the first will was sufficient part-performance of the mutual agreement to give the court jurisdiction to enforce it in equity. The survivor who accepted the benefit under the first testator’s will could not later revoke their own will to defeat the agreed scheme.
The doctrine was confirmed by the Court of Appeal in Re Hagger [1930] 2 Ch 190 (equity acts as soon as the first testator dies and the survivor takes a benefit) and analysed further in Healey v Brown [2002] WTLR 849.
The Floating Constructive Trust
On the first testator’s death, equity imposes a floating constructive trust on the survivor’s assets. The trust is “floating” because it does not attach to a fixed fund: the survivor remains free to spend, consume, or deal with their assets during their lifetime. What the survivor cannot do is give property away gratuitously to defeat the agreed scheme, or make a new will in different terms.
When the survivor eventually dies, the constructive trust crystallises over the net estate — whatever it happens to be — and the agreed beneficiaries can enforce it against the survivor’s personal representatives, even if a later will purports to change the destination of that property.
Revocation Before the First Death
Either party may revoke a mutual will during the joint lifetimes of both testators, provided adequate notice is given to the other. If notice is given before the first testator dies, the other party is free to act on the changed position and the agreement comes to an end.
The point of no return is the first death: once the survivor has accepted the benefit under the first will, the agreement is “clinched by death” (Lord Camden’s phrase) and cannot be unwound by executing a new will or deed of variation.
Proving the Agreement
The agreement must be affirmatively proved. Evidence may include:
- An express written agreement signed by both testators
- A recital in the wills themselves referring to the agreement
- Solicitor correspondence or attendance notes from preparation of the wills
- Credible witness evidence of the agreed terms
Identical provisions alone are not enough — courts have consistently refused to infer a mutual agreement from the mere similarity of two wills.
Why Practitioners Advise Against Mutual Wills
Mutual wills are a blunt and inflexible instrument. The risks include:
- The survivor cannot make new provision for a new partner, a disabled child, or significantly changed family circumstances
- Tax law changes (such as the residence nil-rate band or new IHT pension rules) cannot be responded to
- The floating trust does not prevent the survivor spending down assets — the intended beneficiaries may receive much less than expected
- Litigation to establish the agreement (or resist it) is costly and distressing for families
More flexible alternatives — an immediate post-death interest (IPDI) life interest trust, a protective property trust over the family home, or a discretionary trust with a letter of wishes — achieve similar protective goals while preserving flexibility for the survivor.
FAQs
What are mutual wills and how do they differ from mirror wills?
Mirror wills are two separate wills with identical or near-identical provisions — each spouse leaves everything to the other, then to the same beneficiaries. They can each be changed at any time without reference to the other. Mutual wills go further: the two testators enter a binding agreement not to revoke their wills (or the agreed disposition) after the first has died. Once the first testator dies and the survivor accepts the benefit, equity imposes a floating constructive trust on the survivor's estate to enforce the agreed scheme. Mirror wills are common and flexible; mutual wills are rare, inflexible, and carry significant legal risk for survivors who want to make new provision (e.g. for a new partner or changed family circumstances).
What is the rule in Dufour v Pereira and what did it establish?
Dufour v Pereira (1769) 1 Dick 419 is the foundational authority for mutual wills. Lord Camden held that where two parties make mutual wills under an agreement not to revoke, the execution of the first testator's will is a sufficient part-performance of the contract to render the agreement specifically enforceable in equity against the survivor. The survivor cannot simply revoke and leave the agreed beneficiaries with only a damages claim at law: equity treats the property as held on constructive trust from the moment the first testator dies. The principle was confirmed by the Court of Appeal in Re Hagger [1930] 2 Ch 190 and by the House of Lords analysis in Healey v Brown [2002].
What is a floating constructive trust in the context of mutual wills?
After the first testator dies, equity imposes a 'floating' constructive trust on the survivor's assets. It is described as floating because it attaches to whatever property the survivor holds at death — not to a fixed fund identified at the date of the mutual will. This means the survivor remains free to spend, consume, or deal with their assets during their lifetime (they are not treated as a bare trustee of a fixed sum); but they cannot give the property away gratuitously or change their will to defeat the agreed scheme. When the survivor dies, the constructive trust crystallises over the net estate and the agreed beneficiaries can enforce it against the survivor's personal representatives.
Can mutual wills be revoked before the first testator dies?
Yes. Either party may revoke their mutual will during the joint lifetimes of both testators, provided they give adequate notice to the other. If notice is given and the other party has not yet changed position in reliance on the agreement, there is no binding constructive trust and the agreement effectively ends. The risk arises only after the first death: once the survivor has accepted the benefit flowing from the first testator's estate, it is too late to revoke. After that point, any subsequent will made by the survivor that departs from the agreed scheme will be treated as ineffective against the constructive trust, and the agreed beneficiaries may seek rectification or a constructive trust order.
What evidence is needed to establish that a mutual will agreement exists?
The agreement must be proved — the courts require clear and satisfactory evidence that there was an actual contract not to revoke, not merely a common intention or similar provisions. Evidence may include: (1) an express written agreement signed by both testators; (2) a recital in both wills referring to the agreement; (3) correspondence between solicitors at the time the wills were prepared; or (4) reliable witness evidence of the agreement. The mere fact that two wills are in mirror form is insufficient — many couples make mirror wills with no intention of creating a binding agreement. The burden of proof lies on the claimant seeking to enforce the constructive trust.
What are the practical risks of mutual wills and what are the alternatives?
Mutual wills are widely regarded as a blunt instrument. The main risks are: (1) the survivor cannot make new provision for a new partner, a disabled child, or changed tax circumstances; (2) property values and family needs change over decades — the agreed scheme may become unsuitable; (3) disputes about whether an agreement existed at all are expensive to litigate; (4) the floating trust does not prevent the survivor spending assets, so the intended beneficiaries may receive far less than expected. Alternatives that achieve similar goals more flexibly include: a life interest (IPDI) trust under the first will, vesting the capital in the agreed beneficiaries on the survivor's death while giving the survivor income and a limited power of advancement; a property will trust (protective property trust) to protect the share of the family home; or a discretionary trust with a letter of wishes. Solicitors will generally advise against mutual wills except in narrow circumstances.
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