Parts of the M1 remain closed for repairs after a fire broke out last week.
The Highways Agency has warned that all southbound lanes of the M1 between junctions 1 and 4 will stay shut "until further notice".
The seven-mile section has been shut since Friday following a fire at a scrapyard in Mill Hill, north London.
The agency had hoped to reopen two southbound lanes overnight but has now said that the closures could last all week.
Although two lanes on the northbound carriageway were reopened on Sunday, drivers were told to consider alternative routes as congestion is expected.
The Highways Agency said today: "Engineers worked through the night to try to reopen southbound lanes.
"We are working in very difficult conditions in a confined space and have encountered unexpected difficulties in manoeuvring some of the large and heavy equipment required to reinforce the bridge deck."
Roads Minister Mike Penning said full width of the motorway would not open until "the middle to the end of next week."
He said: "We are working as quickly as possible to reopen lanes on the M1. However safety has to be paramount.
"As a former firefighter I know first-hand the intensity of fires of this kind. I've seen the damage to the bridge and it should not be underestimated how significant it is."
He added that the events that led to the blaze are being investigated by police.
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