A public inquiry into the undercover infiltration of political groups will not conclude until at least 2023, eight years after it was set up, it has been revealed.

The inquiry into undercover policing has already cost more than £10m and was due to finish this year.

It has run into sustained criticism that it was working too slowly, was headed by an old-fashioned and out-of-touch retired judge, and will permit the police to cover up misconduct. The accusations have led to victims of the undercover policing boycotting parts of the inquiry.

Headed by Sir John Mitting, the inquiry has not heard any substantive evidence in public yet. The delay has been caused by the police submitting many legal applications to keep secret the identities of individual undercover officers. The applications were considered over many months at a series of preliminary hearings.

On Thursday, Mitting published the results of an internal review that he undertook after it became clear that the inquiry would over-run its original deadline.

The results of the review prompted dismay from victims of the spying, who said they still feared that the inquiry would not uncover the truth.

Mitting published a timetable for the inquiry that he said would culminate in a final report to the home secretary by December 2023. He said that hearings to examine evidence would start in June 2019 and last two years.

The inquiry is investigating the undercover infiltration of more than 1,000 political groups since 1968, involving, according to Mitting, tens of thousands of documents and the evidence of at least 250 police witnesses.

It was commissioned in 2014 by then home secretary, Theresa May, following a string of allegations about the activities of undercover units, including the disclosure that Scotland Yard had spied on the campaign group fighting for justice for the murdered teenager Stephen Lawrence.

It will also investigate how undercover officers deceived women into intimate relationships, stole the names of dead children to create fake identities and concealed evidence in court cases.

The inquiry is primarily examining the conduct of two units, the Special Demonstration Squad (SDS) and the National Public Order Intelligence Unit (NPOIU), which infiltrated and disrupted political groups.

Mitting said that 171 members of the SDS, including undercover officers and their supervisors, had been identified as having evidence that could be relevant to the inquiry. The SDS operated between 1968 and 2008.

According to Mitting, 84 members of the NPOIU, which existed between 1999 and 2010, could be called to give evidence in some form.

For months, there has been growing discontent from the victims of the inquiry that too many undercover officers were being granted anonymity, which would prevent the truth of what they did being exposed.

In March, at least 60 campaigners and their legal teams walked out of the hearings examining whether the identities of undercover officers should be concealed. Peter Francis, a former undercover officer-turned-whistleblower, joined the boycott this week.

They have not decided to abandon taking part in the inquiry completely. The campaigners have been pressing for Mitting to resign or to sit with a panel, accusing him of being “the usual white, upper-middle-class, elderly gentleman whose life experiences are a million miles away from those who were spied upon”.

In a partial concession to the criticism, he has decided that a panel would join him in 2021, after the fact-finding stage of the inquiry is completed.

“The appointment of additional members to the panel – currently consisting of me, as chairman alone – would impose a heavy cost in both time and money,” he said. “Once the facts have been found, it would be both practicable and desirable for a wider panel to be recruited to investigate and consider the state of undercover policing and to make recommendations to the home secretary for the future.”

A woman, known as Andrea, who was deceived into an intimate relationship by an undercover officer, Carlo Neri, said she was dismayed that the inquiry would finish five years late.

She said: “We have lost years of our lives, due to the harm caused to us by these undercover officers. Our health, relationships and careers have suffered. We want to participate fully, because without the truth we cannot heal. And we want to make sure this state-sponsored abuse cannot happen again.”

Donal O’Driscoll, a campaigner who was spied on, said : “Sadly, it seems to me that Mitting is squandering a huge opportunity, to bring those most deeply affected closer to the process, to build on their insight to help shape the inquiry in a more effective manner, to get to the truth more quickly, more effectively and with more transparency.”

A hearing about the proposed timetable will be held on 18 May.