A British Airways (BA) plane had to return to Heathrow Airport to make an emergency landing after black smoke was seen coming from one of its engines.
Both runways were closed as passengers and crew used emergency chutes.
BA said the Heathrow to Oslo service turned back after a technical fault at 08:43 BST. Witnesses reported seeing flames coming from the engine.
The southern and northern runways had reopened by 11:00 BST and short-haul flights cancelled by BA have resumed.
'Loud popping sound'
In total 192 flights were cancelled from Heathrow Airport following the emergency landing, the majority of which were BA flights, and 22 planes were diverted to other airports.
There are still delays of more than 45 minutes for both departures and arrivals, Heathrow Airport said.
BA said the incident had had "significant impact" on its schedule and delays were expected to last for the rest of the day.
London Ambulance Service said three people on the plane, an Airbus A319 carrying 75 passengers, were treated for minor injuries.
David Gallagher, a passenger on the plane, said: "About eight or nine minutes into the flight there was a loud popping sound, not an explosion but definitely not usual sounds.
"The captain came on very calmly, said he was aware of the situation and that everything was running normally and he was going to run some tests to see what the right course would be.
"Then another five minutes after that there was a loud sound, and this time the right engine was clearly on fire.
"I mean big flames, very visible from the rest of the cabin, and lots of black smoke."
He said a few passengers were visibly upset but the cabin and ground crews were "outstanding and completely calm".
Jez Stamp, from Farnham in Surrey, who was on a plane waiting to take off to Jordan, said: "I could see smoke on the right engine.
"We just saw it come to a halt and fire engines were already there waiting for it and they immediately started hosing the plane down."
Rob Brownell, 33, who saw the plane from Imperial Wharf Station in central London, told the BBC flames were "clearly visible".
He said: "There was this almighty rumbling overhead - I looked up and saw the plane on fire.
"There were flames coming not just from the engine but lapping the wing.
"My initial thought was 'my God this thing is going to blow up'. The wings are full of fuel so I couldn't see how there could not be an explosion."
Normand Boivin, Heathrow chief operating officer, said: "I would like to pay tribute to BA's pilots and Heathrow's fire and airside teams for their quick and professional response."
According to the website planefinder.net, the plane turned over Potter's Bar and Billericay before returning to Heathrow.
Captain Mark Searle, chairman of the British Airline Pilots' Association (Balpa), said: "This was a professional job done by professional people.
"As pilots we spend our whole career training to manage incidents such as this in order to avoid an incident becoming a disaster."