Actress and New York electoral candidate Cynthia Nixon has announced that her oldest child has come out as transgender.

The former Sex And The City star posted the announcement on her Instagram account on Friday to mark the 14th annual Trans Day Of Action.

Her 21-year-old son Samuel Joseph Mozes, who goes by the nickname Seph, was originally named Samantha.

Seph is the child of Ms Nixon and former partner Danny Mozes. She also has a 15-year-old son with him and a 7-year-old son with her wife, Christine Marinoni.

Ms Nixon accompanied Friday’s announcement with a photo of herself with her son in his cap and gown at his graduation earlier this month from the University of Chicago.

The post said: “I’m so proud of my son Samuel Joseph Mozes (called Seph) who graduated college this month. I salute him and everyone else marking today’s #TransDayofAction #TDOA”

Ms Nixon is challenging Governor Andrew Cuomo in New York’s Democratic primary for governor.