Flat doors in Grenfell Tower could only hold back a blaze for half the time they were supposed to, investigators have found.
Experts acting on behalf of the Metropolitan Police tested an undamaged front door from the west London block and found that it could withstand a blaze for 15 minutes - not the 30 for which it was designed.
Seventy-one people died when a fire ravaged the residential building on June 14 last year.
A vast police investigation is continuing into the causes of the fire, Scotland Yard said, as it announced the new findings.
It said in a statement: "The Metropolitan Police Service is conducting a comprehensive investigation into what happened at Grenfell Tower in June last year.
"We have previously described that our forensic examination at the scene would be followed by a phase of offsite testing to be conducted by experts on our behalf.
"As part of this investigation, experts tested a flat front door taken from Grenfell Tower. The door tested was designed to resist fire for 30 minutes but during the test, it was only found to resist the fire for approximately 15 minutes, a much shorter period than expected.
"The forensic examination and testing phase is ongoing and we are not able to comment on the potential impact or otherwise that any test result may have on the overall criminal investigation.
"We have shared this information with the Ministry of Housing, Communities and Local Government as the appropriate Government department, so that they are able to take any action required."
Housing Secretary Sajid Javid is due to make a statement on the subject at 11.30am, it is understood.