The charity is calling for improved guidance from HMRC and the Department for Work and Pensions (DWP) and a wide-ranging review of legislation and practice to achieve “consistency in the definition and treatment of couples for different tax and benefit purposes.”

It also wants an extension of bereavement support to parents who lose a de facto spouse.

Anthony Thomas, LITRG Chairman, said: “Society has changed dramatically over the last few decades. There are now many couples who are not married (nor in a civil partnership), but whose relationships are equally committed. Unfortunately our tax and benefits systems have struggled to keep up with this change.

“Couples are faced with a high degree of complexity in the tax system. Take, for example, the case of a married couple recently separated. There are separate rules that apply to them for eligibility for the new marriage allowance; how long they can continue to transfer assets between them free of capital gains tax and how long transfers between them are exempt from inheritance tax. It may also be difficult for them to “prove” a date of separation to determine when they could make separate claims for tax credits.

“Complexity can lead to mistakes on claims forms and this can have devastating financial consequences. Although two individuals may be clear as to whether or not they are a couple, the authorities may not agree with them and may decide to investigate their status, a compliance nightmare which is exacerbated by the inadequacy of official information accessible to ordinary taxpayers.

“We believe it is possible to have some formal ‘registration’ process with HMRC and the DWP to make an official declaration that a couple has been formed (or separated) to provide some certainty for couples who have made a clear decision to enter into (or separate from) a committed relationship with each other, without forming a marriage or civil partnership.”