The Grenfell Tower public inquiry is running months behind schedule and the conclusions of its investigation into what happened on the night of the fire will not now be published before October.

Sir Martin Moore-Bick, the chairman of the inquiry, had previously told survivors that his conclusions would be ready this spring, but the solicitor to the inquiry, Caroline Featherstone, said on Friday that reaching conclusions was proving “far more complex and time-consuming task than originally anticipated”.

She also said that hearings for the second phase of the inquiry, examining the refurbishment of the west London tower before it went up in flames on 14 June 2017 killing 72 people, would not start until the beginning of 2020. They had originally been expected to start this year.

The delays mean that more than two years will have passed before any official conclusions are drawn about the disaster. They have outraged survivors and the bereaved, who said: “A slow justice is a painful justice.”

Natasha Elcock, a former resident and the chairwoman of Grenfell United, said it was “disgraceful” that the inquiry appeared to have underestimated the complexity of the evidence.

The delay is a second major blow after Scotland Yard admitted in March that no criminal charges were likely until 2021.

“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,” she said.

“It took courage for survivors to give evidence in the first part of the inquiry. We put our faith in Sir Martin Moore-Bick to make change. Six months after hearing our evidence the inquiry is yet to make a single recommendation to keep people safe in their homes.”

The delay appears partly down to the level of fine detail that Moore-Bick wants to include. He said he will provide a “minute-by-minute description of how the fire started, how it spread and what was happening on each floor of the tower”, as well as a detailed description and analysis of what was happening in the London fire brigade’s incident control room and on the ground, as well as the response of the emergency services.

“There is a significant volume of evidence to be reviewed and detailed work to be done to ensure findings are properly tied to all relevant parts of the evidence,” the inquiry said in a statement circulated to lawyers for core participants. “The chairman is seeking to complete the report as soon as possible but needs to ensure he does not compromise its thoroughness and accuracy.”

But Grenfell United voiced frustration that the delay was another sign of the authorities being slow to tackle the problems revealed by the fire.

“It is survivors and bereaved that fought to get dangerous cladding banned,” Elcock said. “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.

“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.”

Moore-Bick also admitted that he is still two months away from sending any conclusions to people who are likely to be criticised in the report – likely to include senior fire chiefs – in order to give them time to respond before publication.

He said the process, known as Maxwellisation after the publisher Robert Maxwell secured a pre-publication right to reply to an official report criticising him, is likely to start only in July.

Families of the bereaved and survivors remain frustrated that the inquiry has yet to tell them where the second phase hearings will take place. They had insisted it move to west London from the legal district in Holborn and that the rooms should be tailored more to their needs than those of the lawyers and the inquiry team.

The inquiry said: “Negotiations on the lease are well advanced and the inquiry hopes to be in a position to announce the venue by the end of May. Building on what the inquiry learned during the first phase of hearings, we will consider all suggestions regarding the facilities and the layout of the new hearing room.”