The girls were last seen on Egerton Road, Twickenham at around 2pm on Saturday, 3 April. They were reported missing later the same day.
Both girls are of slim build and around 5ft 2in tall.
Poppy has blonde/brown shoulder length hair and was wearing a long white coat and knee length riding boots when she was last seen.
Lily has brown hair. She was last seen wearing black trousers and black platform boots.
A missing person investigation is ongoing led by officers from the South West Area Command Safeguarding Unit.
Enquiries so far have established that the girls may be in the Northwood area. They also have strong ties to Hillingdon.
It is believed that they have been in contact with friends as recently as this morning.
Anyone who sees Poppy and Lily should call 999.
Anyone with other information about their whereabouts should call 101 providing the reference 21MIS009223.
In particular, we would urge parents who believe that their children may be friends with Poppy and Lily to find out if they have been in touch and to contact police with any relevant information.