In English law, secret trusts are a class of trust defined as an arrangement between a testator and a trustee, made to come into force after death, that aims to benefit a person without having been written in a formal will. The property is given to the trustee in the will, and he would then be expected to pass it on to the real beneficiary. For these to be valid, the person seeking to enforce the trust must prove that the testator intended to form a trust, that this intention was communicated to the trustee, and that the trustee accepted his office. There are two types of secret trust — fully secret and half-secret. A fully secret trust is one with no mention in the will whatsoever. In the case of a half-secret trust, the face of the will names the trustee as trustee, but does not give the trust's terms, including the beneficiary. The most important difference lies in communication of the trust: the terms of a half-secret trust must be communicated to the trustee before the execution of the will, whereas in the case of a fully secret trust the terms may be communicated after the execution of the will, as long as this is before the testator's death.
Secret trusts do not comply with the formality requirements (such as witnessing) laid down in the Wills Act 1837. Despite this, the courts have chosen to uphold them as valid. Although various justifications have been given for this, they are generally categorised as either based on preventing fraud, or as regarding secret trusts as outside (dehors) the operation of the Wills Act. The first is considered the traditional approach – if the courts do not recognise secret trusts, the trustee given the property in the will would be able to keep it for himself, committing fraud. The fraud theory utilises the equitable maxim that "equity will not allow a statute to be used as a cloak for fraud". A more modern view is that secret trusts exist outside the will altogether, and thus do not have to comply with it. Accepting this theory would undermine the operation of the Wills Act, since the Wills Act is designed to cover all testamentary dispositions. To avoid this problem, one approach has been to reclassify the secret trust as inter vivos ("between the living") but this creates other problems. There have also been attempts to conclude that half-secret trusts rest on a different basis to fully secret trusts, although this has been disapproved by the House of Lords, primarily on practical grounds.
This debate is also of importance when classifying the trust as either constructive or express, a matter of considerable debate. On one view, if the traditional theory is correct, secret trusts are created by the courts, and are thus constructive; if the more modern view is correct, the trusts exist without the court's permission, and are express trusts. However, a secret trust does not have to obey the separate formalities of the Law of Property Act 1925, even when it concerns land and one solution to this problem is to consider them constructive. Some commentators believe that half-secret trusts may fall into a different category to fully secret trusts in this regard.