All on-duty firefighters due to strike in London are being recalled over the blaze at a scrap metal yard being attended by 120 of their colleagues.
There are 20 fire engines attending the scene on Perry Road, Dagenham, east London, where the London Fire Brigade (LFB) say there are around 1,500 tonnes of metal alight.
Cylinders are also believed to be involved and a hazard zone has been put in place in case they explode due to the intense heat.
There are around 1,500 tonnes of metal alight. Picture: fishycol1/TwitterWith black smoke coming from the site, which can be seen from miles around, residents are being advised to keep their doors and windows closed.
Of the 27 fire appliances in operation for contingency, 20 have already been allocated to the Dagenham fire.
LFB said that, because of the major blaze, all striking FBU staff who would normally be on duty were immediately being recalled to their stations.
This was under the agreed "Recall to Duty Procedure" and was voluntary, it said in a statement.
Smoke can be seen from miles around. Picture: JSinghLon/TwitterScotland Yard said police officers and ambulance crews were also at the scene. There are no reported injuries, but there are local road closures in place and motorists are advised to avoid the area if possible.
London Fire Commissioner Ron Dobson said: "We have issued a recall notice and are asking striking Fire Brigades Union members in London to carry on working.
"The FBU have told firefighters not to follow the recall notice. They claim the recall is not valid because there is no risk to life.
"There is no reference to risk of life in the agreed protocols to implement a major incident.
"A major incident can and has been implemented because of the size of this fire and the resources needed to deal with it. By not responding to the recall it is the FBU that is in breach of the agreement."
No one from the FBU was available to comment.
London Mayor Boris Johnson said: "Londoners will rightly want to see the major incident in Dagenham brought to a speedy and safe conclusion.
"I fully support the Fire Commissioner's decision to recall all striking FBU staff who would normally be on duty, and I would urge London's dedicated firefighters to do their utmost to help to bring this fire under control."
It came as firefighters in the rest of England and Wales went ahead with the strike in their dispute with the Government over pensions, on what was expected to be one of the busiest days of the year for the service ahead of Bonfire Night and Diwali celebrations.
Members of the Fire Brigades Union (FBU) walked out at 6.30pm, mounting picket lines outside fire stations until the stoppage ended at 11pm.
A further two-hour strike will be held from 6am on Monday, the day before Bonfire Night.
Fire brigades across the country urged members of the public to put off any firework displays in their own gardens and to switch it to Saturday, or go to an organised event.
The union held a four-hour stoppage last month but called off another strike after it appeared a deal was in sight, but officials said the Government and fire employers had failed to offer any firm guarantees on jobs or pensions as a result of changing the pension age from 55 to 60.
The union fears firefighters will be made redundant if they fail fitness tests, and are unable to find other work in the fire service.
FBU general secretary Matt Wrack said: "It is ludicrous that after two years of negotiations the Government has still not sorted out this mess.
"Firefighters are keen for these issues to be resolved through discussion but the Government won't even listen to its own evidence which highlights that the schemes are unworkable and unaffordable.
"Firefighters want a pension scheme that takes account of the hazardous nature of the job, is affordable and workable for them and for the taxpayer."
South Yorkshire Fire and Rescue said its emergency response service will be "severely reduced" because of the strike.
Chief Fire Officer James Courtney said: "We're satisfied with the contingency arrangements we are putting in place, but our 999 service will still be severely depleted, so we need the public to help keep our fire engines available for real emergencies."
Fire Minister Brandon Lewis said: "This strike action by the FBU is completely unnecessary and does nothing but damage the good reputation firefighters have with the public.
"The public will be baffled by the FBU's course of action when they hear that the deal being offered to firefighters gives them one of the most generous pension schemes in all the public sector."