Around 1,500 passengers were diverted away from Gatwick Airport overnight, because of a combination of fog and maintenance work.
A total of 24 flights were forced to land at Stansted, Luton, Bristol and Heathrow after they were turned away from Gatwick.
Engineers had been carrying out planned maintenance work on the airport's main runway, leaving just the second landing strip open. But poor visibility then led to planes being diverted.
Easyjet, British Airways and Thomson passengers were affected by the disruption.
A spokesman for Gatwick said the main runway reopened at 5.25am. No major delays are expected today although there could be minor disruption to some flights.
"It would be prudent for passengers to check with their airlines just in case," he added.