Military assistance has been called in at Heathrow Airport to help deal with the threat posed by rogue drones.
It is thought Royal Air Force are involved in responding to the latest incident.
Departures from the airport were halted last night for an hour after a drone was spotted.
Police officers were among those who saw it.
Officers investigating at Heathrow say they have learned from the experience at Gatwick before Christmas when drone sightings closed the airport for 36 hours.
Commander Stuart Cundy of the Metropolitan Police said:
"We are carrying out extensive searches around the Heathrow area to identify any people who may be responsible for the operation of the drone."
He added: "Under the Aviation Security Act it is an offence to endanger the safety of an aircraft, anyone found guilty of this offence could face a life sentence."
The Ministry of Defence has confirmed that "specialist equipment" is being deployed to Heathrow, with Defence Secretary Gavin Williamson saying: "At the request of the Metropolitan Police, our Armed Forces deployed to assist and support them.
"Our Armed Forces are always there when needed."
Commander Cundy said: "Military assistance has been implemented to support us."
Brian Strutton, general secretary of the British Airline Pilots' Association (BALPA) said: "This second drone incident in less than a month has shown how important it is that airports invest in drone protection technology immediately."
He added, "An aircraft or helicopter collision with a drone has the potential to be catastrophic and so it's right that Heathrow shut until it was sure flights could take off and land safely again. While it may be frustrating for the passengers who are delayed, it's their safety that must come first."
A Heathrow spokesperson said: "Flights at Heathrow are operating normally, and passengers should follow the usual procedures to check in.
"The use of drones around airports is illegal and dangerous and we continue to work closely with the Met Police on their ongoing investigations.
"We apologise to those passengers whose journeys were unfairly affected yesterday and together with authorities, will continue to monitor our airspace."
The drone sighting came four days after both Heathrow and Gatwick airports reported they were investing millions of pounds in equipment to prevent future flight disruption.
Between December 19 and 21, Gatwick was repeatedly forced to close due to reported drone sightings, causing mass disruption to passengers, with about 1,000 flights affected.
The anti-drone equipment can detect and jam communications between a drone and its operator and was deployed on a roof at Gatwick.
The system, which is said to have a range of several miles, uses four radars to give 360-degree detection in order to identify and track targets.
Following the end of the initial drone-related disturbance at Gatwick, security minister Ben Wallace said:
"I can say that we are able to now deploy detection systems throughout the UK to combat this threat."
On Tuesday, the Government announced a package of measures designed to give police extra powers to combat drones. The exclusion zone around airports will be extended to approximately a 5km-radius with additional extensions from runway ends.
Ministers also announced that from November 30, operators of drones weighing between 250g and 20kg will be required to register and take an online drone pilot competency test.