Fire doors at Grenfell Tower in London could only hold back flames for half the time they were supposed to, investigators have found.
Experts tested an undamaged door from the block and discovered it could only withstand a blaze for 15 minutes - not the 30 minutes for which it was designed.
The Metropolitan Police announced the developments as it continues a "comprehensive" investigation into the tragedy last June which left 71 people dead.
Three glazed fire doors provided by the same company - believed to be Manse Masterdor - were found to offer less protection than guidelines recommend.
It is understood they were installed as front doors to flats in the west London block between 2011 and 2012.
Synseal said it did not know which type of doors had been tested by investigators and that it "did continue to make a variety of fire doors previously supplied through Manse Mastedor," but added "all the door styles had been tested and certified to British Standards produced by BSI Group."
The company took over Manse Masterdor's trade and operating assets two years after the doors were installed.
Manse Mastedor remained a separate company and was renamed Litchfield Investments Limited in 2014.
It is now in the process of being wound up.
Those affected by the tragedy were alerted to the findings in a letter from the Metropolitan Police.
Natasha Elcock, a survivor from the tower and a representative of the bereaved and survivors' group Grenfell United, said: "It's shocking - first the cladding and insulation then the doors. Who knows what else is putting people's lives at risk?"
"It's time people's lives are taken more seriously, and that includes everyone from every walk of life."
British Housing Secretary Sajid Javid said the government would carry out further tests, but there was "no evidence that this is a systemic issue", and the risk to public safety was low.