The American Airlines jet was forced to make a U-turn after setting off for the UK capital from Philadelphia on Sunday.
It had to divert in Boston at Logan Airport where 10 of the 12 crew members were transported to hospital.
One passenger told local media she though it was a traveller letting off wind, saying: “I kept smelling a funny smell, and I wasn’t quite sure what it was.
“I thought someone had an upset stomach, so I ignored it.”
A second passenger added: “It smelled kind of more like a sort of burning smell I would have thought. But more like a strange burning smell.”
The Airbus A330-300 aircraft landed in Boston at 11.48pm local time on Sunday – an hour after departing.
Data on FlightRadar24 showed the plane circling off the east coast of the US as the incident unfolded before it diverted.
A statement from American Airlines confirmed: “American Airlines flight 728 from Philadelphia to London Heathrow diverted to Boston Sunday evening after a possible odour was reported in the cabin.
“10 of the 12 crew members asked to be transported to a local hospital for evaluation; all were later released. No customers experienced any issues.
“All 154 customers were provided with overnight accommodations in Boston, and will continue their journey to London today.”
American Airlines added that its maintenance team was “looking into the issue” which caused the smell.