According to Sussex Police, Hayley Williams, 42, was reported missing from Brighton on Sunday (July 19) with her three daughters - seven-year-old Frankie Langley, and Betsy and Bobbie Langley, both aged five.
A police spokesman said: "We are growing increasingly concerned for their welfare and are appealing for anyone who sees them to get in touch.
"Hayley is white, 5ft 6ins, of heavy build with brown hair. The three girls are also white, and all with brown hair. It is not known what they are currently wearing."
Hayley is known to have access to a light blue Mercedes car, and has links to the Twickenham and Isle of Wight areas, police said.
Anyone who sees them or who has any information as to their whereabouts is asked to call 999 quoting serial 927 of 19/07.