Highways England spokesman Stuart Thompson told the Associated Press that safety checks had proven "very complex", particularly on the part of the bridge that looms over the highway's eastbound lanes.
The westbound carriageway remains closed.
Highways England has said that the road closure will last until at least tomorrow.
Motorists are facing mounting misery after a pedestrian bridge collapsed onto the M20 closing the motorway in both directions.
One motorcyclist was injured and taken to hospital with suspected broken ribs.
Images from the scene show two trucks trapped under the rubble while video footage shows the top of one lorry's trailer smashed off and debris scattered across the tarmac.
My instinct was just to drive through. My wife saw the bridge falling and there was a big noise.
Reports said vehicles were queuing for more than four-and-a-half miles along the road where some13 million drivers are expected to take to the road between Friday and Monday.
The reopening was several hours later than planned and a precautionary 50mph limit is in place in both directions past the bridge.
Kent Police said it is treating it as a "major incident" and confirmed that officers were at the scene, along with Kent Fire and Rescue Service and South East Coast Ambulance Service.
Witness Andy Sunnucks, 24, said: "We are about 15 cars back on the same carriageway as the collision".
The bridge came down between junctions three and four shortly after noon on Saturday.
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