The British Airports Authority, which operates the airport, has launched an internal inquiry into why the crack appeared. A spokeswoman said the full facts were still being gathered.
The crack appeared at the world's busiest airport in the late afternoon of July 6, as it approached one of its busiest periods. The runway was shut at 6.40pm and did not reopen until the next day.
BAA admitted last night that 24 flights were cancelled and others delayed by up to an hour. But it rejected reports that 150 flights were disrupted, and that "several holes" appeared on the runway.
It said "a long thin crack" had suddenly appeared in the middle of the runway, caused by the widening of an expansion joint. Such joints are fitted at intervals along the runway to allow the surface to re act to changes in temperature.
Passengers were told about delays during the course of the evening but not the reason for them. Heathrow is now approaching the busy holiday season and staff are under strict instructions to make sure there are no hitches.
A BAA spokeswoman said the circumstances of the crack were "extremely unusual" and that she had never heard of a similar situation before. "But things like this can occur without warning, like a gasket going on a car."
BAA said teams of runway checkers constantly scour Heathrow's two runways, searching for signs of debris, and examining surfaces.
BAA said it was confident that no accident would have occurred, "but we cannot afford to take any chances with the safety of passengers and crew. That is why we took the decision to close the runway".
Emergency repair work went on well into the night. The spokeswoman said the expansion joint became detached from the asphalt layer above it. The defect was discovered during a routine inspection.
BAA said the condition of the two runways was immaculate and that its inspection system was devised to ensure that defects were discovered immediately.
But some staff are worried that the incident occurred at all. One said last night: "Last week was hot, but the temperatures have not been consistently high over a long period. It has given us some cause for concern."
Heathrow handles an average of 1,250 flights a day, peaking in the early morning and evening with returning flights from the UK and the rest of Europe. It can handle about 80 takeoffs and landings an hour.
Last year it handled around 60m passengers, making it by far the busiest international airport in the world, with over 90 airlines serving 200 destinations.
Its busiest day was August 2 last year when 209,297 passengers used the airport. The most popular destinations are New York, Amsterdam, Paris and Dublin.
BAA says that Heathrow generates more than £3bn in wages alone each year for the economy, supporting 200,000 jobs directly and indirectly. Some 57,000 of these are at the airport.
