Friends or cousins of the same sex wishing to pass on assets after their death free of inheritance tax may be surprised when their tax adviser suggests they tie the knot.

From December 5, individuals of the same sex can have their relationships legally recognised in the form of a civil partnership. While civil partnerships were intended to give gay and lesbian couples legal recognition, accountants and lawyers say they do not preclude any two individuals of the same sex – provided they are not direct family members – from forming a civil partnership to take advantage of the significant tax breaks.

The civil partnership extends the same rights enjoyed by husbands and wives, the biggest tax advantage being the ability to leave assets to a surviving partner without them incurring inheritance tax.

“There is no express requirement in the Act for the two individuals to live together, to be of any particular sexual orientation or to be in a sexual relationship,” said Simon Rylatt, tax and financial planning partner at Boodle Hatfield.

However, under the rules parents, siblings, grandparents, aunts and uncles will be barred from registering each other as civil partners, as will half-brothers and sisters and those in adoptive relationships. Cousins will be able to make use of the rules.

Tax advisers say that civil partnerships are not to be entered into lightly, due to the rights and responsibilities of the legal union. But as record numbers of estates are moving above the inheritance tax threshold of £275,000, forming a civil partnership may become a mainstream option for individuals looking to pass on assets tax efficiently after their death.

“It would always be pointed out to clients now that there are advantages to getting married in that they can pass assets between each other,” says John Whiting, president of the Chartered Institute of Taxation. “If there is a couple of friends who jointly own a house, then forming a civil partnership will be drawn to their attention. Civil partnerships will be put in the toolbox of solutions offered to clients.”

However, legal experts say there are plenty of downsides to entering into a civil partnership purely for tax gain.

“If one of the partners wanted to leave all or part of his/her estate to children (or to anyone else) then on death there is a risk that the surviving partner might try to thwart that,” says Julian Washington, partner with Forsters solicitors.

There could be further heartache down the line, if a civil partner wants to marry or end the relationship.

“Elderly couples entering into civil partnerships for tax advantages may be shocked to learn that they can only terminate the relationship by a process equivalent to divorce called dissolution,” says Vivien Hardy, family law partner at Boodle Hatfield. “As with divorce, no application can be made before a year has elapsed and the court has to be satisfied that the civil partnership has broken down irretrievably. This process can take several months and can be expensive.”

A spokesman for HM Revenue & Customs said that the department does not look at the motivation for couples entering into marriage and this would be the same for civil partnerships.