Passengers said that the incident – which happened on Monday – was "scary" and felt like a "horror movie", with travelers shouting that they were unable to breathe.
A passenger on board, Lucy Brown, told CNN that the smoke had been thick with a metallic, chemical odor, adding: "We covered our mouths with our clothes. We don't know why oxygen masks didn't release."
"Passengers started shouting that they couldn't breathe," she said.
"Flight BA422 from Heathrow to Valencia experienced a technical problem with the landing to Valencia. All of the customers were safely evacuated by our crew and met by the airport's emergency services," a BA CNN spokesman said in a statement.
There were 1[ads1]75 passengers on board the flight. A spokeswoman confirmed to CNN that three clients were taken to hospitals following the "as a precautionary event" and have since been discharged.
"Oxygen masks are used for loss of pressure. It is not standard procedure for oxygen masks to be used when there is smoke in the cabin and at low altitude," a spokesman for BA said to CNN.
In a statement posted on their website, the British Airline Pilots Association (BALPA) said they believed the pilots landed the aircraft "wearing full oxygen masks and goggles." Oxygen masks for passengers were not deployed because, "unlike oxygen systems for pilots and cabin crew, they are not designed to be used in smoke events as they mix oxygen supply with air."
British Airways said there were two pilots and six cabin crew on board.
"What scared us on the plane was no tannoy system working to communicate what was happening, and it took 15 minutes for emergency doors to open," Brown told CNN.
"The safety of our customers and crew is always our top priority. In addition to our onsite team, other British Airways team members have arrived in Valencia to help our customers and our local airport partners with everything they need," said the BA spokesman .
"In this case, pilots and crew appear to have done an excellent and very professional job of getting this aircraft safely on the ground in very difficult circumstances, and safely evacuating all passengers without serious injuries reported," he said.
BA did not answer any further questions from CNN.