Grenfell Tower's design made it chimney

Urban Search and Rescue officers from London Fire Brigade inside the Grenfell Tower in west London. Photo / AP

Investigators believe the unusual design of Grenfell Tower created a chimney effect that led to the fire engulfing the building in minutes.

A well-placed source has told the Telegraph that cladding placed over "triangular" shaped concrete columns may have created a void that sucked up the flames.

Ten columns run up the sides of the building and a further column is at each corner. Investigators now believe that when those columns were fitted with cladding, they could have created an air gap that acted as a chimney.

Videos of the blaze clearly show the flames spreading upwards far faster than they spread sideways. Investigators are looking at whether fire breaks were built in to the newly clad columns. Given the speed the fire spread up the side of the building that appears unlikely.

The development came as cladding samples from 34 high-rise buildings in 17 local authorities were found to have failed safety tests after the Grenfell Tower disaster, which killed at least 79 people.

The British Government has admitted that it failed to order compulsory testing of combustible insulation material used in tower blocks in a move branded "ludicrous" by safety experts.

Grenfell Tower was fitted with combustible cladding and flammable insulation.

The insulation was fitted between the original concrete pillars and the external aluminium cladding with a plastic flammable core used for rain protection.

The source said: "The investigation is not just looking at the materials but the design of Grenfell Tower. The triangular shape created by the cladding has created a void and the flames have shot up the side. The greater the gap, the more it acts like a chimney. It is one line of the investigation."

Grenfell Tower was built in 1974 and the concrete pillars that ran up its side are unusual. That may give some crumb of comfort to residents in other tower blocks that have also been clad.

Experts said that perhaps of even greater concern is the failure to order the testing of the insulating material behind the panels, which is potentially even more flammable. The danger of insulating material was confirmed by early tests on the panels used to clad Grenfell Tower ordered by the Met Police, as part of their criminal investigation into the disaster.