Concern is mounting over marriages of convenience - an unintentional loophole in the new Civil Partnerships Act.
Experts in the family and matrimonial team at Birmingham law firm Mills & Reeve are the latest to predict it could prove a serious issue.
The Act, which allows gay couples to "marry", could also see heterosexual friends of the same sex jump on the bandwagon to take advantage of the available tax breaks.
Among these is the exemption from inheritance tax, which married couples enjoy and which was extended to same sex couples registering their partnership under the new laws.
According to Matthew Hansell, who heads the private client team at the firm, this can mean significant savings.
He said: "A married couple or a couple who have entered into a civil partnership can leave their estates to their partner in their will. In the event that one dies, the other takes the estate free from tax.
"But if the couple are not wed, or have not entered into a civil partnership, then they will only inherit the first £275,000 of the estate tax free, and must pay the Exchequer 40 per cent of the remaining value.
"These days, when you factor in the cost of a home, a pension pot, savings and any other assets, you can see how easy it is to exceed the minimum threshold."
Mr Hansell says heterosexual couples, whose relation-ship is based on friendship, can now register their part-nership in a bid to cheat the Chancellor of inheritance tax.
He said: "Take two elderly women, one of whom has become a carer for her friend.
The friend may choose to demonstrate her gratitude by leaving her carer some money in her will. But if the estate is sizeable, she may not want the Exchequer to get the benefit of it. In which case, the friends may register their partnership to take advantage of the tax breaks."
If there is no will, a spouse or a civil partner will have rights under the Intestacy Act. But if they are not married or have not registered their relationship it is hard to make a claim on a dead partner's estate.
Similarly, a registered partner is automatically regarded as the next of kin. Mr Hansell said: "In the case of the elderly couple, the sick friend may prefer to trust decisions on her medical care to her long-time carer, rather than her family. A civil part-nership agreement between the two would make this automatic."
It would be hard for family members to challenge the validity of the "marriage".
He noted: "I am sure the Government never intended that the law could be used in this way - it was aimed at the gay community. But they have created a loophole which it will be very hard to close."