European air traffic co-ordinating agency Eurocontrol said the problem was caused by an "issue with radar displays".
"Everything was done to keep trains moving where possible, and last night hundreds of staff were out fixing the damage and repairing the railway ready for today".
Having earlier warned against making non-essential journeys, Network Rail later urged passengers to check for the latest service information before they travel.
Heathrow, which saw a record-breaking 37.9 Celsius (100 degrees Fahrenheit) yesterday, said: "Flights are now arriving and departing at Heathrow, and we are supporting NATS to resolve the issue as quickly as possible".
Woodstock 50 to be free event
But it wasn't until today that Woodstock 50 organizers formally released all artists from their contracts. As of publishing, no other artists have publically pulled out of the festival.
Man surprises wife with an Amazon Prime package birthday cake
You know you order from Amazon too often when your partner gives you a birthday cake shaped exactly like a giant Amazon package. Take a closer look at the hilariously realistic cake ahead. "In that moment, I knew my husband 'gets me", she told CafeMom .