A £1,000 reward is on offer for information after copper cables were cut in Croydon, leaving hundreds of customers without telephone and internet services.
The damage was caused on Tuesday, February 19, on Southbridge Place at about 10.30am.
Hundreds of BT Openreach customers lost services and it is understood this is not the first incident to have happened in the borough.
Crimestoppers and Openreach began working together in February 2011, to combat the malicious damage and the disruption of services to customers.
BT Security General Manager for Cable Theft, Simon Davies, said: "Cable thieves seem to be targeting Croydon once again with other incidents having occurred close-by recently.
"If you have any information on activity happening around a manhole in Southbridge Place, mid-morning on Tuesday, please contact Crimestoppers now and help safeguard your community services."
Regan Hook, Crimestoppers Regional Manager for London appealed for people to come forward.
She said: "This is such a needless crime and it is infuriating for all those affected."
Anyone with information should contact Crimestoppers anonymously on 0800 555 111 or using a secure online form at crimestoppers-uk.org.