By SHARON CHURCHER in New York and PAUL HENDERSON in Houston

Last updated at 10:53 17 February 2008

Friends: Sharon Bush (left) with Jack Worthington's sister, Nancy

The Texas tycoon who claims to be John F Kennedy's secret love child had an affair with President Bush's sister-in-law, The Mail on Sunday can reveal.

New York socialite Sharon Bush says she conducted a romance with financier Jack Worthington, 47, after the break-up of her marriage to Neil Bush, the US President's multi-millionaire brother.

Sceptics have derided Mr Worthington as a "wacko" or an opportunist for his extraordinary claim about his parentage.

But Mrs Bush – who is the mother of supermodel Lauren Bush – strongly defended him.

She says he was brought up to believe his father was the respected Houston school administrator to whom his mother was married for 47 years.

But then his mother allegedly told him his real father was America's assassinated 35th President, to whom Mrs Bush says he bears a "stunning" physical resemblance.

"When his mother gave him the information, just a few years ago, he was shell-shocked," Mrs Bush said.

"He is a fine man, highly ethical, a man of high integrity.

"He only wants to know the truth."

Mrs Bush, 55, said she dated Mr Worthington following her acrimonious divorce in April 2003 from Neil Bush, 53. The couple had been married for 23 years.

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Family resemblance?: Jack Worthington (left) and John F Kennedy

President Kennedy, who was gunned down in Dallas in November 1963, regularly cheated on his wife, Jackie.

His conquests are known to have included Marilyn Monroe.

There is no public record of him ever meeting Mr Worthington's mother, who is identified in Texas birth archives as Mary Evelyn Bibb.

Critics have suggested her son is either a "fantasist" or a fortune-seeker who hopes to make a financial killing from his story.

Supermodel: Sharon Bush's daughter, Lauren

However, Mrs Bush said he had been determined to keep his quest for a Kennedy DNA sample, which he hopes will substantiate his claims, in "complete confidence".

A reporter for Vanity Fair magazine, which has been preparing an article about his claims, allegedly leaked his name to the media.

Mr Worthington contends that he is infuriated by his new notoriety.

As a successful mergers and acquisitions expert, who worked for major American companies before founding an internet banking firm, he insists he has no motive to lie.

"It's outrageous," he said.

"Vanity Fair approached me.

"They had to woo me for over six months to work with them on their story, and I co-operated only on their assurances that my anonymity would be protected."

His mother, who was widowed last year, still lives in his family's impressive house in an affluent district of Houston.

She did not respond to repeated requests for comment.

The Mail on Sunday also attempted to interview Mr Worthington's sister, Nancy – who has been photographed with Sharon Bush on the US social circuit – but police outside her home refused to allow us to approach her.

Neighbours and friends of the twice-divorced businessman were sharply divided in their opinions.

Ken Lindsay, who lives across the street from Mr Worthington's mother, said: "The claims come as a shock because his mother is not that type of woman at all.

"She and her husband were always very good, friendly and caring neighbours right up until the time he died last year, and I have the utmost respect for her.

"She was so loyal and good to her husband throughout his decline in health. Everything was very normal."

However, Mr Worthington's former business partner, Doug Vandermotter, took his side.

He said: "I met his mother. She seemed like a normal, older woman.

"She never mentioned JFK and nor did Jack, so this has all come from out of the blue.

"It sounds kind of wild, but Jack is not a fantasy artist.

"I don't think he'd say he believes he is Kennedy's child unless he believed it."