The Highways Agency said it hoped to reopen the M1 by 0500 BST on Monday Parts of the fire-damaged M1 into London may now not reopen fully until Monday, the Highways Agency has said.
The seven-mile stretch of road between junctions one and four was closed both ways after a scrapyard fire on Friday.
One lane of the northbound motorway was reopened on Saturday evening. Officials said the southbound section would stay shut “until it is safe to be reopened”.
The closure comes as thousands of fans head to the capital for the London Marathon and a FA Cup semi-final.
The fire, at a scrapyard under the M1, also caused some nearby railway lines to be suspended.
A Highways Agency spokesman said urgent repairs were needed before the motorway could be fully reopened.
He said officials hoped to have the road open again by 0500 BST on Monday.
The spokesman added: “The viaduct does need to be reinforced before it can carry the weight of traffic and we are putting in supports so we can reopen the road as soon as possible.
“At the same time we are continuing our investigations to determine what remedial work needs to be carried out.”
On Saturday, the motorway closure led to delays on the A1, A40 and A406 as motorists took alternative routes, AA Roadwatch said.
The closure coincided with one of the biggest weekends in London’s sporting calendar.
About 36,000 runners are expected to take part in the marathon later, with thousands of supporters due to line the route of the race.
Thousands of fans are also expected at the 90,000-capacity Wembley Stadium for the FA Cup semi-final between Bolton Wanderers and Stoke City.
About 40 firefighters were called to tackle the blaze, which broke out at a scrapyard site close to Scratchwood Services and Ellesmere Avenue in Mill Hill early on Friday.
A number of gas cylinders were involved and a hazard zone was set up in case they exploded.
About 50 people living in properties near the scrapyard were temporarily moved.
Motorists have been advised to check the Highways Agency website for the latest traffic information.
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