Police have launched a CCTV appeal to find a mother reported missing with her two young children.
Cheryl Wilkins, 31, and her sons Dylan Wilkins-Barnes, four, and Leo Wilkins, two, were seen in Canvey Island at 9am on Wednesday June 28, Essex Police said.
Officers believe the trio are together, and that 31-year-old Damien Barnes, who is the children's father, may also be with them.
Ms Wilkins and Mr Barnes are not the legal guardians of the boys, who were taken without consent.
Superintendent Nick Morris said: "Cheryl has not been in touch with her family since Wednesday and we are concerned for her and the boys' welfare.
"There is no indication that any harm has come to them and our inquiries are ongoing to find them.
"I'm asking Cheryl to please get in touch so that we can know you and the children are safe.
"I know this is a difficult time for you and we want you to have the opportunity to speak to us so we can help you and the boys.
"Our priority is Leo and Dylan's welfare first and foremost.
"They have been away from home and their friends and must be unsure of what is happening.
"I am sure all those who know you and the boys are worried about you and want to know you are all okay."
Ms Wilkins and her children were seen on a train travelling from Benfleet to Barking on the day they were reported missing and both Ms Wilkins and Mr Barnes have links to St Albans and South Ockendon.
Officers believe they may be in the Hertfordshire area and that they may be living in bed and breakfast accommodation, hotels, caravan or camping sites.
Police have released CCTV images of Ms Wilkins and her children at Barking railway station on June 28, and in Furtherwick Road in Canvey Island.
The images of Mr Barnes are from St Albans on June 29.
Anyone who has seen them is asked to contact Essex Police on 101.