The former world champion was travelling from Geneva, Switzerland to the British capital when he complained about pains in his chest and then passed out.
Air stewardesses gave the 27-time grand prix winner first aid, and paramedics took Stewart straight to London's Hillingdon hospital when he landed at Heathrow Airport.
The star's son Paul tells the BBC, ""He did pass out and as a precaution he was taken to hospital to be looked at. I've just spoken to him in hospital. He seemed absolutely fine but obviously something happened and we need to establish what, but I'm on my way to see him right now. I should think he'll just be checked over and hopefully it will be nothing more sinister than that.""