Kara Kennedy, the 51-year old daughter of the late US Senator and Democratic Party icon Ted Kennedy, died of an apparent heart attack, her family said.

Kara Kennedy “suffered an apparent heart attack following her daily work out”, the Edward M. Kennedy Institute for the US Senate said in a statement.

Ms Kennedy, who had battled lung cancer, died suddenly late on Friday at a Washington-area health club after her daily workout.

The oldest child of Ted Kennedy, who lost a battle to brain cancer in August 2009, Kara Kennedy is survived by her two teenage children, her mother, her brothers Ted Jr and Patrick – a former Democratic Congressman who retired in January – and her stepmother and stepsiblings.

Kara’s father Ted Kennedy seemed destined for the highest office after his brothers were assassinated – first President John F. Kennedy in 1963, then senator Robert F. Kennedy, as he campaigned for the Presidency in 1968.

But personal scandal and a reputation for drinking and womanizing got in the way, and he settled in for a long career in the US Senate.

Kara Kennedy, who helped run her father’s 1988 senate re-election campaign, accepted the Presidential Medal of Freedom from President Barack Obama on behalf of her ailing father days before his death in 2009.

“My daughter was my best friend,” Kara’s mother Joan Kennedy told ABC News.

A filmmaker and television producer who had once worked at a Boston television station, Kennedy was a producer for Very Special Arts, a non-profit organisation founded by her aunt Jean Kennedy Smith.

She was also a member of the National Organisation on Fetal Alcohol Syndrome advisory board.