Millions of couples who have chosen to avoid tying the knot are at risk of falling into complex legal battles to get a share of a home they've lived in all their lives or the wealth they've helped build.
The numbers of cohabiting couples in the UK has risen to five million, according to the latest figures from the Office for National Statistics (ONS). But unmarried couples don't have automatic legal rights to inherit from each other.
'Common-law marriage' - where you live together without making it official - is a widely held misconception that will not protect the deceased's partner. The only way to ensure your partner doesn't lose out when you die, if you're not married or in a civil partnership, is to write a will.
Here, Which? reveals how one bereaved partner found out the hard way and how to make a legally binding will that leaves no room for doubt.
Where there's no will, there's no way of ensuring your partner gets anything when you die, if you haven't made it official.
No matter how long you've been together, lived together or even whether you've had children together if you weren't married or in a civil partnership, your partner has no automatic legal rights to inherit when you pass away.
'If you want to protect your partner, it can be more important to have a will if you are unmarried than if you are married or in a civil partnership,' says James Buchan, a Which? Legal lawyer.
Such a reversal of fortune can come as a hammer blow at a time of loss and grief.
Your partner may face a huge struggle to prove they should get property and maintenance, not to mention the wider strife and inter-family division that can ensue.
In some cases where there has been no will, bereaved partners have been forced to bring a financial claim against their own children and other members of the family.
Dennis found this out the hard way after partner, Linda, (not their real names) died suddenly without a will.
Although they had lived together for almost 40 years, Linda's entire estate including prime London property and substantial investments, all in her name only, passed to her nearest surviving relatives - nieces and nephews whom she barely knew and would not have wanted to inherit anything.
Dennis was left out in the cold. Not even Linda's generous pension looked likely to pass to Dennis because they'd not been married. To top it all, Dennis didn't even have the right to register Linda's death or arrange her funeral.
As with any unmarried partner, to inherit anything Dennis faces a struggle and an uncertain outcome. However, after seeking help through Which? Legal, Dennis found out he had options.
He could appeal to the nieces and nephews to give up some or all of their inheritance, but that would be entirely at their discretion.
Dennis could also try to prove that Linda maintained him and that he will need that maintenance to continue. If the relatives don't agree, Dennis has a strong case (he had been living in the property with Linda), but a court will have to decide, and it could either award a lump sum or merely allow Dennis to stay on for the rest of his life.
Alternatively, Dennis could take legal action showing he had funded maintenance of the home and therefore has some rights over it. However, in other people's cases, this has been a difficult argument to make and prove.
With the pension, Dennis must demonstrate how long he had known and lived with Linda before he gets some part of her retirement funding. But this is at the discretion of the pension provider.
What you need to put into your will if you're living with a partner will depend on your circumstances, particularly if children are involved.
To look after the survivor, it's important the will includes:
Where there are children involved, matters can become more complicated. It's vital to include instruction on the age that children can inherit.
As a will is even more important for unmarried couples than those in marriages or civil partnerships, it's vital that it's drawn up and updated by a law professional to be enforceable.
This needn't be as daunting or as expensive a task as it may seem.
Once you're ready to write one up, you can approach a lawyer or online service.
The most basic option allows you to fill in your will online in your own time with helpful hints and tips. The review service gets your will looked at by one of our wills experts, which gives you extra peace of mind that you will have a legally binding will.
The service also includes support for those who are unfamiliar with using digital services.