Lawyers are seeing an extraordinary rise in the number of disputes over wills. There has been a threefold increase since 2004, according to City law firm Wedlake Bell, while East Anglian and Midlands solicitor Mills & Reeve now handles so many that it has three specialists working on them. London firm Withers is seeing 'a huge increase' and now has 10 partners dedicated to the work.

Why? Specialists largely agree on the main causes. First, far bigger legacies are being left, pushed up by rising house and share prices over the past decade. Despite recent property price falls, the average house is worth about £100,000 more than it was in 1998. Second, there is less of a social stigma in taking relatives to court in such disputes, and people are even feeling encouraged to do so by high-profile cases. Third, modern family structures (especially through second marriages and cohabitations) are creating complications, with children from a first marriage or long-finished relationship, for instance, sometimes feeling left out. Fourth, people are living longer, succumbing more to dementia and leaving wills that can be challenged on the basis of mental capacity.

This is a difficult area of the law where judges can feel forced to give decisions that are correct in legal terms, but seem unfair. This year, for instance, three Court of Appeal judges said David Thorner had a 'strong moral claim' to inherit his cousin's £2.3m Somerset farm on which he had worked diligently, for nothing, for 28 years, seven days a week.

His cousin Peter had written a will leaving David the farm, but it was not found when he died in 2005. David said Peter repeatedly promised him the 400 acres, both by his behaviour and in conversations, but the judges felt there was insufficient proof. Peter was deemed to have died intestate, so his estate went to his sisters and closer blood family.

No legal principle insists wills should be fair. 'You can leave your estate to whoever you want,' says Fay Copeland, head of Wedlake Bell's contentious trust and probate team. A parent can legitimately exclude a child, or leave money equally to all offspring, even if one child did much more for that parent than the others.

Lawyers expect the surge in will disputes to continue. 'A huge swathe of the population has entered into the bracket whereby disputing the will makes economic sense, even though litigation is expensive,' says Paul Hewitt of Withers.

In 2007-08, at least 34,000 people left estates worth more than £300,000 - about one in 23 people who died. Going to court can cost as much as £150,000, says Wedlake Bell; the risk is that if you lose, you may have to pay the legal costs of the other side as well.

The main grounds for challenging a will are that a person lacked capacity (if, for instance, they suffered from dementia); that the will was not properly executed (if, perhaps, the two witnesses were not present at the same time as the testator signed); that the testator was unduly influenced (for example, by a malevolent carer); or that there was a 'want of knowledge and approval' (perhaps when a person signed a will drafted by someone else without reading it).

Some of these arguments will very often be run together.

Another line of attack for someone who was financially dependent on the person making the will (a live-in partner or child, for instance) is the Inheritance (Provision for Family and Dependants) Act 1975. If a man forgot to update his will when he left his wife for his girlfriend, the girlfriend could use the act to say she should be provided for.

The best advice is to try to settle. As well as costing a great deal of money, court decisions do not tend to provide 'win-win' solutions. At the start of a dispute, relationships can break down totally between estranged family members. But most cases settle when clients start paying the ongoing legal costs.

Copeland is used to organising 'family meetings' to try to negotiate settlements. 'At the start, we encourage both sides to read out a statement, or to outline their concerns and what they want to achieve,' she says. 'That tends to humanise the situation and take a lot of heat out of it.' Compromises usually follow.

But people writing wills can take steps to minimise the likelihood of disputes. If, for instance, a parent decides to leave no money to their wealthiest child on the grounds they do not need it, Copeland suggests talking it through first with the family and also leaving a 'letter of wishes' attached to the will. She says: 'Something as simple as that can head off a dispute.'