If you make a will with your partner leaving everything to them first and to your children second, then you'll almost certainly believe your wishes are going to be honoured.

After all, that’s the point of the popular ‘mirror wills’ set-up; you both have identical documents that leave everything to your partner if they are still alive and your children if they are not.

They have the same stipulation, which means that whoever dies first inherits the lot and then the children or other loved-ones get the estate afterwards.

Except that, with a mirror will you can’t actually guarantee that the money will then be left to your children or other loved ones if your partner decides otherwise.

They could even change their will while you’re both still alive and there is no rule that you would have to be informed – they might be drawn up at the same time but they are not actually linked legally.

And, in a time when blended families are becoming increasingly common, growing numbers of people are discovering that their loved ones’ last wishes are either not what they expected or not being honoured.

The Financial Times reported in August this year that there had been a 36% surge in inheritance disputes brought to the High Court in 2016. That’s likely to be the tip of the iceberg as most inheritance claims and disputes are resolved privately.

So what are the dangers of mirror wills and is enough done to ensure people know about them?

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A very sad example

Stuart Herd is an active and vocal campaigner against the risks involving mirror wills. He has first-hand experience of the risks involved, having never received a penny of his father’s estate due to a perfectly legal process.

His widowed father, William, had remarried in his late 60s and Stuart was pleased to think that he would have company and support in his old age.

William and his new wife Dorothy drew up mirror wills that provided for both the remaining spouse and the children each had from their previous marriages.

When one died, the estate would pass to the surviving partner. On their death, it was to be shared equally between Stuart and one other person: Dorothy’s son.

Yet on her death more than a decade after his father's, Stuart discovered that Dorothy had changed her will and left everything to her son. Instead of the £150,000 he had expected, he has nothing.

He told personal finance website loveMONEY: “What hurt so much was that it wasn’t only my father’s estate but my mother and my grandparents’ estates too. And it’s ended up with a family that don’t even know who they were.

“The hardest thing has been that I really wanted to provide a start for my daughter. It seems so wrong that someone can come along and take an estate.”

When his father’s widow died, Stuart discovered that she had changed her mirror will four years previously, leaving a letter that claimed he had not bothered contacting her for 10 years.

That’s an account that he disputes, saying he even has proof that they had been in touch even just days before the new will was created.

However, even without that claim, the truth is that Dorothy was entirely within her legal rights to amend her will and disinherit her husband’s son.

That’s a key reason why Stuart is such a determined campaigner. “What I feel there should be is some sort of warning on the document, warning you that the will could be changed at a later date. I don’t believe a lot of people understand this.

“I don’t know if my father was even aware that this could happen. Most people have their wills prepared in the expectation that their intentions will be honoured.”

[Read more: Why you need a will and how to get one written]

How can one partner’s wishes be ignored?

Mirror wills are often recommended because they are so cost effective; they are essentially the same document and so the couple can get a discounted rate compared to drawing up two individual wills.

That discount can encourage people to draw up their wills and that can only be a good thing - more than half the UK adult population does not have a will.

The Co-Op Legal Services website says that mirror wills can be a great way to save money. However, it is very transparent about the risks:

Its guidance states: “Either party is free to change their will at any time. It is important to know that although a couple’s wishes may be identical, their respective wills are theirs alone. Therefore trusting your partner is vital because if you decide to change your will, you do not have to tell your partner.

“However, just remember, they are not obliged to tell you either! This could be significant particularly if, after your death, the surviving partner re-marries or has further children with someone else.

“If you die first, you may have left your estate to your partner so, if they subsequently decide to change their will, they could pass on your assets to people you do not want.”

That’s not the only potential issue. The legal services firm April King warns that if you leave everything to your partner and they then require time in a care home then much of the estate could be whittled away before it passes to your children.

What are the alternatives?

There are alternatives to mirror wills but it's vital to take legal advice. Tasoula Addison, wills and trusts lawyer at Gorvins Solicitors, explains that people are often not told about the alternatives to mirror wills.

She says: “One solution that we often encourage due to today’s modern family is ‘life interest trusts’ in people’s wills. [These] specify how assets (property, finances etc.) are divided when the remaining spouse passes away.

“Because it is a trust stipulated in the original will of the first to die it is legally binding and so cannot be changed unless everyone agrees, [for example] the children who were originally named.

“However if assets include property then it would require you and your partner to become ‘tenants in common’ rather than ‘joint tenants’.

Another option is to leave gifts directly to children in your will rather than all to the spouse, which often happens in mirror wills.”

Ms Addison highlights that there are other kinds of trusts such as discretionary trusts where the spouse and children or whoever you choose are named as trustees and all have to agree how things are split between them.

However, these are not particularly common.

She says this is because “many people think that their spouse will automatically follow their wishes and don’t take into account that circumstances can change when they die,” which is what happened in Stuart’s case.

Often, people want their will planning to be straightforward and may think that trusts provide an extra layer of complication. However, Ms Addison says that ensuring everyone’s legacy is protected is the best way to keep things simple.

It’s also important to have conversations, she urges. “When making wills and planning for the future, we always suggest including the members of the family you intend to leave assets to.

“This is to encourage frank and honest conversations within families to avoid disputes and family-feuds in later life.”

What you can do now

Whatever the risks of mirror wills, it is still better to have one in place than to hope your wishes are somehow honoured without a will at all.

But ultimately, it’s never been more important to seek guidance when estate planning and to have serious and open discussions with loved ones about your wishes.

Otherwise, you risk being left, or leaving your children, like Stuart – with more questions than answers and considerable regrets.