A material routinely used in hundreds of thousands of council homes causes a greater fire risk than its traditional counterpart, a BBC London investigation has found.
The revelation comes days ahead of the inquest into the deaths of six people who died when fire ripped through Lakanal House in Camberwell, last July.
PVC windows were implicated in a fire which gutted a Croydon block in 2007
Across London vast numbers of buildings have seen traditional wooden window frames replaced with PVC plastic, including a block in Croydon which burned to the ground in 2007.
These replacements had also been made at Lakanal House, in Camberwell, south London, where six people were killed when a fire ripped through the building, last July.
Experts have blamed the speed that the fire spread there on a windows as well as a replacement facade for the building made of plastic.
And now the BBC has seen the results of a test that compared PVC windows to those with wooden frames.
Safety analysts, the BRE, found wooden frames gave nine minutes more fire resistance than that of PVC.
Chris Houston, a fire risk consultant, said: "Any fire engineer will tell you PVC burns easier than wood.
"The fire resistance of PVC isn't some highly debated subject in the industry - everyone would agree that it has zero fire resistance."
ANALYSIS
The BBC has highlighted the following missed chances to prevent deaths in Lakanal House:
A 2000 parliamentary report warning about dangers from plastic external cladding being ignored
Southwark Council knowing Lakanal House was a fire risk but not taking proper measures
Southwark Council failing to carry out a legally required fire risk assessment that could have warned of the danger
London Fire Brigade telling people to stay in their flats because they believed properties would offer an hour's fire protection - they provided 20 minutes
LFB failing to prosecute Croydon council after a similar fire in 2007
While some fire experts would contradict Mr Houston, what is not in doubt is that councils across London use PVC instead of wood because installation is cheaper.
BBC Freedom of Information requests sent to every London borough show 212,000 council houses now have PVC windows - half the total stock.
It is the policy of 15 out of 32 London boroughs to exclusively install PVC windows.
Now BBC London has learned of the striking similarities in the Lakanal disaster to that of the fire which destroyed the block in Croydon, on Christmas Day 2007.
The official report into that fire blamed incorrect installation of PVC windows.
PVC was used to fill gaps around the window - when the fire began it melted, causing the block to burn down.
Nobody was killed, but resident Andrew Byrne was among several who lost everything.
He said: "I saw the fire enter my flat through the window space. The fire was dripping down through the window.
"The windows in the block had been replaced (with PVC). The previous windows had been made of wood."
In Croydon 90% of council properties have PVC windows, the highest figure in London.
There are other strong parallels between the Croydon fire and the Camberwell tragedy.
Along with replacement PVC windows in place, neither had had a valid fire risk assessment - meaning both councils had not carried out basic safety checks.
Dudley Mead, deputy leader of Croydon Council, said: "Hindsight is a wonderful thing.
"We were just setting out [on doing risk assessments] at the time and did not rate it as high risk.
"We got external advice as to what caused this fire and made amendments to all blocks in this style."
Lack of prosecutions
After the Croydon fire, the decision was taken by London Fire Brigade (LFB) not to prosecute the council.
Now experts say the failure to prosecute was another "missed opportunity" to prevent the Lakanal tragedy.
Arnold Tarling, a surveyor with 22 years' fire risk experience, thinks prosecution would have prompted all councils to ensure they had risk assessments in place for all their blocks.
He said that could have prompted Southwark Council to carry out its outstanding check on Lakanal House.
The council has said it will not comment on the claims until after the official investigation report into the Lakanal fire is published.
Footage from Croydon in 2007 shows flames entering PVC windows
Mr Tarling explained: "It would have been a warning - and protected lives."
Of the BRE's report, Mr Tarling said: "The PVC windows failed long before the timber windows.
"PVC windows expand [in fire] and when that happens internal linings bow.
"When they bow they allow fire and smoke to get between them and the frame. With timber frames that doesn't happen."
But the British Plastics Federation, which represents plastic manufacturers, pointed to another BRE report from 1987 which concluded "PVC window frames do not pose an unacceptable risk".
Spokesman Philip Law said: "PVC is part based on chlorine which acts as a fire retardant. This means it is slow to ignite.
"Once the source of ignition is removed PVC stops burning whilst wood continues to burn."
It took Mr Byrne a year to get back on his feet after the Croydon fire destroyed his flat.
He said: "I carried around a lot of anger. I certainly had post-traumatic stress."
But the six victims in Camberwell lost more than their possessions.
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