The M4 in west London has reopened in both directions a week after it was closed for emergency repair works, the Highways Agency has said.
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 on Thursday.
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 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 is the main route from London to Heathrow Airport and the West of England. 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.
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: "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."