A 78-year-old woman from Los Angeles plans to come forward tomorrow with what she claims is incontrovertible evidence that Thurmond, who died in June at the age of 100, was her father.

Thurmond, the country's longest-serving senator, began his political career as a Southern Democrat, but broke with the Democratic party to run for the presidency on a segregationist platform in 1948. "On the question of social intermingling of the races, our people draw the line," he said.

At the time his alleged daughter, Essie Mae Washing ton-Williams was in her early 20s. Now a retired school teacher, Ms Washington-Williams says she has financial receipts, cashiers' cheques and personal notes that prove Thurmond recognised the paternity, including a thank you note for a Father's Day card.

Her lawyer, Frank Wheaton, says she revealed her identity "at the urging and encouragement of her children" to establish her family history.

"I did not want anybody to know I had an illegitimate father," Ms Washington-Williams told the Washington Post. "My children convinced me to tell the truth. I want to finally answer all of these questions _ that have been following me for 50 or 60 years."

Mr Wheaton says she waited until Thurmond, who later switched to the Republican party, had died before coming forward. "She didn't want to do anything to damage or potentially damage his political career or his family," he said.

Thurmond's estate, which has been valued at around $200,000 (£115,000), is currently being settled, but Mr Wheaton insisted this had no bearing on the timing of the revelations.

"Let's be emphatically clear: We are not looking for money. We are merely seeking closure by way of the truth for Essie Mae Washington-Williams, so her children have an opportunity to know from where they come; whether those ancestors are black or white matters not. It is part of our American history."

Thurmond would have been 22 when he fathered Ms Washington-Williams by 16-year-old Carrie Butler, who lived in the poor, black district of Edgefield, South Carolina, and worked in Thurmond's parental home. Ms Butler was unmarried and her neighbours helped feed and clothe the baby until the child was taken at six months old to live with an aunt, according to the Washington Post.

Ms Washington-Williams first met Thurmond around 1941, when she visited Edgefield aged 16. Her mother was suffering from an untreatable kidney disease and insisted on introducing her to her father, she said.

Ms Washington-Williams said Thurmond called her a "very lovely daughter". She said: "I was very happy. I knew I had a father somewhere, and it was wonderful to meet him."

A year before he stood for the presidency championing the separation of the races he met his black daughter again.

Mr Wheaton said: "He asked her directly, 'How does it feel to be the daughter of the governor and not be able to tell anyone about it?' She said it felt fine." Mr Wheaton says Ms Washington-Williams's existence was never a secret in the Thurmond household. But few were ready to admit it.

Over the years that followed there were persistent rumours that he was the father of a black child.

But Ms Washington-Williams said Thurmond denied them and he insisted she was a friend.

Yesterday, the Thurmond family's lawyer, Mark Taylor, said in a brief statement: "As Strom Thurmond has passed away and cannot speak for himself, the Thurmond family acknowledges Ms Essie Mae Washington-Williams' claim to her heritage. We hope this acknowledgment will bring closure for Ms Williams."