All departures at Gatwick Airport in West Sussex were suspended around 9 a.m. as a safety precaution when the balloon was spotted, The Daily Telegraph reported.
Five flights were delayed for more than 10 minutes while the balloon was move out of the way, an airport spokeswoman said.
When the balloon was moved, planes were cleared for takeoff, the spokeswoman said.
"The airport is not closed and arrival flights are still landing, but departing flights were suspended for safety reasons," she said.
Sussex police said the balloon then drifted off and police have not been able to relocate it.
"We are aware and are trying to assist in locating the balloon," a police spokesman said. "We've not been able to do anything about it because it's really foggy. We've had officers looking into the sky and they can't see anything."
Liz Meek, editor of aviation magazine Aerostat, said hot air balloon pilots are asked to carry maps and be aware of where controlled airspace starts.
"They are also able to radio air traffic control if they feel they are unable to avoid it but would try and land as soon as possible, certainly before getting anywhere near the actual airport, let alone the runway," she said.