Ken Jordan, who became a rich man through his business, Central Linemarkings Ltd, died aged 75 and left every penny to his new partner, the glamorous cousin of his wife, the High Court heard.

He cut his “secret daughter” out of his will just months before his death in August 2012. Instead he left everything to Bernice Elliott, 65, whose relationship with Mr Jordan triggered the breakdown of his 50-year marriage.

Now Ruth Simmonds — his 58-year-old daughter from a relationship before he married — says he may not have been in his right mind when he disinherited her.

Mrs Simmonds would have got £100,000 from her father’s estate under an earlier will and claims that is the last valid one he ever signed.

The court heard she used to give lifts and visit him at the Sunrise care home, in Bagshot, Surrey, where he spent the last year of his life.

But his brother-in-law and solicitor, Richard Mumford, said Mr Jordan “kept his cards close to his chest”.

He told the court: “Ken never mentioned Ruth to me at all. I knew of her existence, but he never told me he had given anything to Ruth.

“I have no idea why he changed the will, but he knew his own mind and that is what he wanted to do.”

Mr Jordan had two children with his wife, Vivienne. The couple’s marriage broke down around 2008 when Mrs Jordan discovered her husband was in a relationship with her cousin, Miss Elliott, the court heard.

Mr Jordan continued to live with his wife at their home in Frimley, Surrey — despite her filing for divorce — but later moved to Sunrise after being diagnosed with terminal cancer.

After the split, Mr Jordan changed his previous will to cut out his estranged wife and children. Instead, he left everything to his lover — except £100,000 each to his two sisters and Mrs Simmonds.

But he changed his mind again in January 2012, after a falling out with his family, and this time left lock, stock and barrel to Miss Elliott.

Lawyers for Mrs Simmonds argued there were “real doubts” over Mr Jordan’s mental ability to make a valid will in the months before he died.

Her barrister, James Weale, pointed out that there was no independent medical evidence detailing what his state of mind was at the time.

He told the court: “Real doubts have been raised about the deceased’s capacity and his knowledge and approval... Those would have been dispelled if he had contemporaneous assessment from a doctor.”

But Mr Mumford said Mr Jordan knew what he was doing when he made Miss Elliott his sole heir.

Oliver Hilton, for Miss Elliott, argued the 2012 will was valid and that Mr Mumford, as Mr Jordan’s brother-in-law and solicitor, was “best placed” to judge his mental state.

Judge Edward Murray reserved his ruling until the new year.