A businessman who left £8.2m to the Conservative Party has had his will overturned in the High Court following a successful challenge by his son.

The court was told that Branislav Kostic, a Serbian born businessman who settled in England in the 1960s, drew up a will in 1974 making his son Zoran the sole beneficiary.

However, he then began to develop a mental illness and suffered from delusions. He believed that there was an international conspiracy of “dark forces” which included close members of his family.

In the 1980s he wrote to Prime Minister Margaret Thatcher saying she was the only person who could protect the free world from bestial monsters. He later cut his son Zoran out of his will and left his money to the Conservatives.

When Mr Kostic died in 2005, Zoran challenged the will saying his father was suffering from a mental illness and that he would never have behaved in the way he did if he had been of sane mind.

The court ruled that the original will of 1974 should be validated so that Zoran was reinstated as the sole beneficiary.

This is an extreme case both in terms of the huge sums involved and the extent of Mr Kostic’s mental illness. However, it illustrates that it is possible to successfully challenge a will if it seems to be unfair or out of character with the known wishes of the deceased person.

Sometimes a will is challenged because someone suspects that the deceased person was subjected to undue influence by their spouse, or perhaps a relative or carer. There may be occasions when there is even a suspicion that a will has been forged.

Second relationships started later in life often lead to a will being challenged when one of the partners dies. For example, a second wife may challenge a will if she feels her husband has not provided for her properly. From the other point of view, a person’s grown up children will often make a challenge if they suspect that their deceased father has left too much to his second wife.

It is important that everyone should make a will and ensure that it is properly drawn up by a solicitor. However, people should also remember that it is possible to take legal action if they feel that something untoward has taken place in the making of the will.