One lane of M1 southbound near Mill Hill 'could reopen'
Steel supports are being placed under a bridge on the M1 in the hope of reopening one lane on the southbound carriageway, which has been closed after a fire.
A seven-mile section of the M1 in north London, between junctions 1 and 4, was shut after the blaze in a scrapyard in Mill Hill on Friday.
Roads minister Mike Penning said the heat caused concrete to explode.
Highways Agency engineers said it was a "miracle" the bridge did not collapse.
The southbound route may not be fully reopened to traffic until the end of the week, engineers said.
Two northbound lanes were reopened over the weekend, which was one of the busiest of the sporting calendar in London with two FA Cup semi-finals at Wembley and the London Marathon.
Football supporters from Manchester, Bolton and Stoke were forced to take diversions on the M40.
'Very unsafe structure'
Highways Agency had planned to reopen at least one southbound lane on Monday morning but its plans were delayed.
A statement said: "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."
Mr Penning, a former firefighter, said the fire in the scrapyard, which was beneath the motorway, had seriously damaged the pre-cast concrete bridge and engineers have been working round the clock since the incident.
He told BBC Breakfast: "What damaged the infrastructure of the elevated section here is the heat.
"It was so intense that some of the concrete has exploded and some of the steel has expanded which has made the concrete drop away.
"So it is a very unsafe structure at the moment."
Highways Agency workers have already put up 200 tonnes of steel support pillars across the width of the carriageway under the bridge and will be adding to it to make the structure safe enough for some traffic.
Mr Penning said: "We will do everything we can to get the southbound stretch open today, but sadly the inside lane on the hard shoulder was so badly damaged it is beyond repair and we'll have to do remedial work on that later on.
"We're looking to open two lanes south today and we will try and operate a contraflow using the hard shoulder on the northbound carriageway to have three lanes north and three lanes south."
Describing the fire under the bridge as unprecedented, he added: "This particular yard, I understand from the local authorities, has been operating for nearly 30 years with no problems, and I've fought fires under bridges, particularly under railway bridges in the arches, in the past and I've never seen the sort of physical damage to the structure of the bridge seen here."
His comments came after Transport Secretary Philip Hammond told BBC1's Politics Show on Sunday about plans to review what businesses should be allowed to operate under motorways.
Paul Watters, the AA's head of roads and transport, said tens of thousands of motorists had been affected by the shutting of the motorway.
He said: "We do wonder why perhaps it can't be opened to light traffic.
"Its also the mixed messages we are getting. I was looking at the Highways Agency's website last night (Sunday) and it was suggesting that two lanes would open southbound from 11am today, but that's been put back quite clearly."
He also welcomed plans for a review announced by Mr Hammond saying: "We can't afford to have links like this out for this length of time."
About 40 firefighters were called to tackle the blaze at the scrapyard which broke out at a site close to Scratchwood Services and Ellesmere Avenue in Mill Hill early on Friday.
About 50 people living in properties near the scrapyard were temporarily moved and some railway lines were suspended for hours.
Motorists have been advised to check the Highways Agency website for the latest traffic information.