Experts acting on behalf of the Metropolitan Police tested an undamaged front door from the west London block.
They found 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 14 June last year.
A vast police investigation is continuing into the causes of the fire, Scotland Yard said, as it announced the new findings.
A statement said: “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 off-site 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.”
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 peoples lives at risk. It’s time people lives are taken more seriously and that includes everyone from every walk of life.
“People’s homes must be made absolutely safe for them and their children. The Government should have improved regulations after previous fires. We can’t listen to anymore excuses.
“Grenfell United will not stop campaigning until the lessons of Grenfell are learnt. Nothing can bring our loved ones back, but we must make sure a fire like this never happens again.”
It is believed Manse Masterdor was the manufacturer of Grenfell Tower’s fire doors, which were installed between 2011 and 2012.
Since then, the company’s trade and operating assets were taken over by Synseal, which continues to trade under the name Mastedor.
Synseal said the products in question were no longer being sold.
The former owners changed the company’s name in November 2014 from Manse Masterdor to Litchfield Investments Limited, public records show.
In January of this year it was agreed Litchfield Investments would be wound up voluntarily. Liquidators were called in to oversee the process and distribute assets.
A spokesman for Synseal said: “The doors were fitted in 2011 and 2012. There was a two-year gap between Manse Masterdor installing the doors and Synseal taking over.
“In terms of products, when Synseal came to the company it was because it wasn’t performing very well, they changed lots of products, lots of specifications of doors.
“In terms of the doors used in Grenfell Tower, those would not be the same doors that are manufactured today.”