Trains were unable to run between Shenfield and Whitechapel while Network Rail responded to a fire alert, creating major disruption on the other side of the capital.
Heathrow confirmed the road tunnel to Terminals 2 and 3 had reopened shortly before 11.30am on Tuesday, and trains on the Elizabeth Line were able to resume running to the airport within the hour.
A Heathrow spokesperson apologised for the disruption and told the Standard the Central Terminal Area tunnel was forced to close due to a technical issue.
National Highways had warned that the M4 Heathrow Spur was closed and told travellers to head towards Terminal 5 instead.
The disruption comes days after Heathrow and other airports across London had to ground flights following a technical glitch at air traffic control.
No planes were able to take off for around 20 minutes at airports across the country following an issue at a control centre in Hampshire on Wednesday, creating a backlog of delays during the busy summer period.
This week, Heathrow also revealed details of its £49 billion plan to expand and modernise the airport.