United flight #UA941 is declaring an emergency and diverting.
The Boeing 767 departed at 16:25 GMT from London Heathrow to Newark.
But 45 minutes into the flight, while flying over Dublin, the crew decided to turn away.
The pilots squawked 7700 (general emergency).
The flight is operated by a Boeing 767-300 (reg. N685UA) which is 21 years old.
UPDATE #1 17:40 GMT
Emergency services are arriving at London Heathrow.
UPDATE #2 17:45 GMT
We have confirmation #UA941 is returning to Heathrow due to a technical issue.
UPDATE #3 17:53 GMT
The Boeing 767-300 is preparing for a heavy landing on runway 27R at London Heathrow.
UPDATE #4 17:56 GMT
The Boeing 767 is returning due to fuel control imbalance.
UPDATE #5 18:03 GMT
#UA941 landed runway 27R at London Heathrow.
UPDATE #6 18:04 GMT
The aircraft vacated the runway, being inspected by fire trucks.
UPDATE #7 18:15 GMT
The Boeing 767 is now parked at remote area of the airport for technical checks.
