Officers were called to Canary Wharf station just before 9am today after passengers on the Tube reported the alleged offender.
Despite the scare, which caused significant delays to trains on the line between London Bridge and Stratford, police did not track down their suspect.
The man was also allegedly seen on board a Central Line train earlier today.
A British Transport Police spokesman said: “Officers were alerted to reports of a man carrying a knife on board a Central line train service on Friday, 6 March, shortly before 9am.
“The man is thought to have travelled on a Jubilee line service to Canary Wharf . Officers searched a train at Canary Wharf, but no one was found matching the description.”
A spokesman for Transport for London tweeted: “No service between Stratford and London Bridge and minor delays on the rest of the line while the police carry out an investigation.”