In the last few moments, a Virgin Atlantic Airbus A330 bound for Washington from London Heathrow has declared an emergency.

VS21, operated by G-VKSS, is seen turning back towards the UK capital over Galway, and at the time of writing, is at FL290 currently.
At this stage, it is unclear what the cause of the emergency is, with more updates to follow in due course.
This is a developing story.
Updates From Virgin Flight VS21 from London to Washington…
UPDATE #1 @ 2044 UK time – Virgin Atlantic flight VS21 originally bound for Washington, is just entering the UK coastline.
UPDATE #2 @ 2046 UK time – G-VKSS, the Airbus A330-300 operating this flight, is 12.4 years old at the time of writing, having been delivered to Virgin Atlantic back in February 2011.

UPDATE #3 @ 2051 UK time – Data from @RadarBoxCom shows that Virgin Atlantic VS21 bound for Washington initially is at 378 knots, cruising at FL290. It will be descending soon back into London Heathrow.
Diverting back to Heathrow due to engine vibrations https://t.co/nkQiY8rhqN
— Flight Emergency (@FlightEmergency) June 16, 2023
UPDATE #4 @ 2052 UK time – Courtesy of @FlightEmergency, it is understood the cause of the emergency is due to engine vibrations. VS21, operated by the Virgin A330 to Washington initially has begun its descent back into London Heathrow just now.

UPDATE #5 @ 2055 UK time – At this time, it is unclear what runway the Virgin Atlantic Airbus A330 will use, especially with the aircraft suffering engine vibrations. Priority for the 09s potentially?

UPDATE #6 @ 2059 UK time – Virgin Atlantic flight VS21 is descending through 20,000 feet to the north of Bristol.

UPDATE #7 @ 2104 UK time – Virgin Atlantic VS21 is descending through FL120 at present, with Runway 09L/R looking to be the assigned runway for the emergency.

UPDATE #8 @ 2108 UK time – Virgin Atlantic flight VS21 bound originally for Washington, is nearly on the final approach for the 09s at London Heathrow.
UPDATE #9 @ 2110 UK time – VS21 opting for Runway 09L, it seems. More to follow on this still.

UPDATE #10 @ 2117 UK time – Virgin Atlantic Flight VS21, originally bound for Washington, has arrived safely back onto Runway 09L at London Heathrow. This continues to be a developing story.

UPDATE #11 @ 2119 UK time – VS21 has safely vacated Runway 09L following reports of massive engine vibrations on the Airbus A330-300, G-VKSS.