Friday fire so badly damaged an elevated section that it may be middle to end of week before motorway can be fully reopened
A seven-mile stretch of the M1 into London may not reopen fully until towards the end of the week because an elevated section has been so badly damaged by a fire in a scrapyard, according to transport minister Mike Penning.
The delay means the country's main south-north trunk road could still be partly closed for the Easter getaway.
Two northbound lanes were open by Sunday afternoon and two southbound ones were expected to open on Monday morning, but it might be "the middle to end" of the week before all lanes are open, the minister said.
The closures, between the motorway's junctions 1 and 4 at Brent Cross and Elstree, disrupted the travel plans of thousands of people, including football fans travelling from Manchester, Bolton and Stoke for two FA Cup semi-finals at Wembley over the weekend and others visiting for the London marathon.
Appealing for motorists to continue to seek alternative routes along the M40, A41 and A1, Penning said he was sure government agencies would look at how many scrapyards and stores were built under roads following the effects of the blaze early on Friday morning.
The reinforcement of the elevated section has involved putting new supports under the road. One lane northbound was opened on Saturday evening but drivers will continue to suffer significant delays and diversions for some time.
Penning said: "We have to build a new bridge underneath the existing bridge so we can open more lanes and get traffic moving safely."
In an interview with Sky News, he said the hard shoulder was particularly badly damaged. "It is a pre-cast concrete bridge and the heat has damaged it so badly that the steels are exposed that are part of the infrastructure of the bridge."
Penning said: "We have to make sure it's safe for the motorist and safe for our workers underneath the bridge."
He would not comment on the particular incident that caused the motorway closure, saying a police investigation was under way and he did not know what had been stored there. But he was sure the Environment Agency and Department for Transport would "look at these sorts of places around the country".
He added: "I am sure there are hundreds and hundreds of these sorts of scrapyards and storage places. We don't know exactly what was stored here – the local authority will be involved on Monday morning – but the key is to make sure we get as much of the motorway open as soon as possible, but safely."
Penning said: "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.
"Even as lanes do reopen I would urge drivers to still consider using different routes to avoid congestion on what is already a busy route in and out of London."
Scores of firefighters tackled the original blaze which led to the evacuation of nearby homes and disrupted rail services. The London fire service has said a number of gas cylinders were involved and a hazard zone was set up in case they exploded in the heat.
The Manchester United manager, Sir Alex Ferguson, earlier described the decision to play two semi-finals involving four northern clubs at Wembley as "quite incredible". Supporters of a high-speed rail link to the north are also likely to point to the weekend road chaos as evidence for their cause.
An AA spokesman said Penning's remarks were "a bit worrying" and lessons must be learned. He said there were also hundreds of railway viaducts where inflammable materials were stored under arches.
The transport department said it would be looking at the scope of any future motorway checks over the next few days.