The publication of the Grenfell Tower Inquiry's first report has been delayed until October.
Core participants, including survivors and those who lost relatives in the deadly blaze, were expecting it would be released this spring.
But in a letter obtained by the Press Association they were told on Friday that writing the report "has proved to be a far more complex and time-consuming task than the inquiry had originally anticipated".
It said: "The chairman will be in a position to write to the Prime Minister with his final report after the parliamentary recess, for publication most likely in October."
The report follows the first phase of the inquiry, which is limited to what happened on the night of June 14 2017 when fire ripped through the west London tower block, killing 72 people.
Caroline Featherstone, solicitor to the inquiry, said in the letter that plans for the second phase are continuing and it is still due to go ahead in January 2020.
Natasha Elcock, chairwoman of Grenfell United, the group for survivors and bereaved families, said: "It's disgraceful the inquiry have underestimated the complexity of the evidence that was produced in Phase 1 and have further delayed the report until autumn.
"That we are only finding this out now, when we were expecting the report to be published ahead of the two-year anniversary. Shows how they continue to disregard survivors and bereaved through this process."
The letter said the report will set out in detail what happened on the night of the blaze.
"That involves an almost minute-by-minute description of how the fire started, how it spread and what was happening on each floor of the Tower," it said.
"It also involves a detailed description and analysis of what was happening in the incident control room and on the ground, as well as the response of the emergency services and relevant organisations."
Inquiry chairman Sir Martin Moore-Bick has previously said he does not consider it appropriate to make interim recommendations ahead of the report.
But some survivors have become frustrated that safety recommendations, such as abandoning the "stay-put" policy for buildings over 10 storeys, are so far yet to be implemented.
Ms Elcock said: "It is survivors and bereaved that fought to get dangerous cladding banned.
"There is still no change to the stay put policy, people are living in tower blocks without sprinklers and social housing residents across the country are still be ignored and mistreated.
"It appears, despite everything survivors and bereaved said in the weeks after the fire, the inquiry has woefully underestimated the catastrophic complexity and scale of the failures that led to Grenfell.
"They also continue to woefully underestimated the needs of bereaved and survivors."
She added: "We want the inquiry and the criminal investigation to be thorough and to get to the truth, but there must be no more delays.
"We are living in a limbo, increasingly frustrated and we need to know there will be some resolution soon. A slow justice is a painful justice for all of us."