A British Airways (BA) plane had to return to Heathrow Airport to make an emergency landing after black smoke was seen coming from an engine.
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 have since reopened but BA have cancelled all short haul flights until 16:00 BST.
Emergency services were standing by while the plane was evacuated.
London Ambulance Service said three people had been treated for minor injuries.
BA said the Airbus A319 was carrying 75 passengers and a “full investigation” would be conducted into the incident.
The Metropolitan Police attended the emergency and tweeted: “Police at Heathrow assisted in the response to the emergency landing. It is not being treated as a terrorist incident at this time.”
Witness John Coghlan told the BBC he saw a “black smoke trail” from the engine of a British Airways Airbus A319.
He said: “[It was a] safe landing and passengers evacuated by chutes on runway.”
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.”
Earlier, London Fire Brigade tweeted: “One crew from Heathrow fire station is assisting Heathrow Airport’s fire service with an aircraft fire.
“We believe the fire is now out.”
Rob Brownell, 33, saw the plane from Imperial Wharf Station in central London and 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.
“It left a black smoke trail in the sky and there was a smell of aviation fuel afterwards.”
Normand Boivin, Heathrow chief operating officer, said: “Heathrow is now fully operational and the airport is returning to normal.
“I would like to pay tribute to BA’s pilots and Heathrow’s fire and airside teams… for their quick and professional response to today’s incident.”
Mark Freeman, the airport’s duty manager, said: “Incidents like this are extremely rare but the team trains constantly to ensure we are prepared when they occur.”
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.
“BALPA representatives will be assisting the pilots involved in this incident and providing whatever support they need.”
Earlier, the airport said 23 flights had been diverted, including two to Manston Airport in Kent, as a result of the incident and advised travellers to contact their airlines to see if their flight had been affected.
The incident is being investigated by the Air Accidents Investigation Branch (AAIB).