In a statement, a Heathrow Airport spokesperson said the drone was first sighted in the airport's vicinity at approximately 5:05 p.m. local time (12:05 p.m. ET) and that the airport was "working closely with the Met Police to prevent any threat to operational safety."
Departures were stopped as a "precautionary measure" during the investigation, the spokesperson said.
Some flight departures had resumed by 6:20 p.m., according to the aviation tracking website Flight 24.
Shortly after, a Heathrow Airport spokesperson said that while departures had resumed, they were continuing to monitor the situation and apologized to passengers who had been affected by the "short suspension."
The disruption at Heathrow comes just weeks after drone sightings brought Gatwick Airport to a standstill for 36 hours, affecting the plans of about 150,000 passengers during the busiest travel period of the year. The perpetrators have not yet been caught.
In the wake of Gatwick's drone disruption, UK airports including Heathrow and Gatwick invested millions of pounds in anti-drone technology, according to British media reports.
On Monday, the UK's Department for Transport said the government was moving forward with plans to give police new powers to "tackle drones' misuse and abuse." Under new legislation, police will be given additional powers to land, seize and search drones. Exclusion zones around airports where drones are banned from flying will be extended and drone operators will be required by law to register beginning in November 2019.
Heathrow is currently the world's seventh-busiest airport in terms of passenger numbers, with 78 million in 2017.