The M4 motorway in west London has reopened in both directions a week after it was closed for emergency repair works, the Highways Agency said this morning.
The road, which is expected to be one of the busiest routes during the Olympic Games, was shut last Friday between junctions 2 and 3 after cracks were discovered on a viaduct.
But it was reopened to traffic just after 5.30am after engineers completed the "complex" repairs, officials said.
The works to the bridge supports of the Boston Manor Viaduct were due to be completed yesterday.
A Highways Agency spokesman said: "An exceptionally delicate and difficult operation to repair cracks in the Boston Manor Viaduct on the M4 has been successful and the motorway has now reopened between junctions 2 and 3 to all cars."
The hairline crack was found on a highly stressed section of bridge supports by engineers who had been carrying out other repairs on the busy section of road.
"The Highways Agency closed the motorway last Friday night because in entering the final stages of the repair work, a new crack was found in a highly stressed part of the bridge supports," the spokesman continued.
"This required a unique repair solution involving the manufacturing bespoke specialist steel plates and their installation over the crack.
"The crack then had to be removed to prevent further damage to the structure. This intricate part of the operation took several hours - even though the crack was a hairline one."
The spokesman added: "The crack was discovered during an ongoing repair programme.
"Work on that continues and for this reason vehicles weighing 7.5 tonnes or more will still be restricted from using the motorway between the junctions. A diversion route remains in place via the A4 and A312 dual carriageways. Drivers are thanked for their patience during this complex work."
The M4, which is the main route from London to Heathrow Airport and the West of England, was initially closed between junctions 1 and 3, causing misery for thousands of motorists.
It is expected to be one of the busiest routes during the Games because it links London and the Olympic Park in Stratford with Heathrow.
The motorway will officially become part of the Olympic Route Network on Monday, with one lane of each carriageway reserved for competitors, officials and sponsors.
Drivers have been warned the ORN will include temporary changes including suspended turns, stopping and loading restrictions and traffic signal alterations.
Labour MP John Woodcock described the reopening of the M4 as a "huge relief", but said ministers needed to come out and confirm they were totally happy that the road is safe to use.
He told BBC Breakfast: "The whole country wants the Olympics to be a huge success so it is a huge relief that the road is open because of the chaos it would have caused with athletes arriving on Monday to travel to the Olympic Village."
The shadow transport minister added: "There are real questions as to why this crack was found so late in the day as works have been going on there for some time.
"Given the scale of the problem, ministers need to come out and say they are completely confident the crack is fixed."