The spycops public inquiry is at risk of having its scope and powers reduced after the Home Office opened a consultation on whether the judge-led probe should continue in a different format. Victims of undercover surveillance say that limiting the inquiry now would amount to a cover-up and prevent full exposure of decades of misconduct.

Set up in 2015 and originally slated to conclude within three years, the inquiry has been running for more than a decade and is expected to continue for another two years. It has so far cost £144m. Retired judge Sir John Mitting has led the investigation into the Special Demonstration Squad, a Metropolitan police unit that infiltrated political groups between 1968 and 2008, and is due to examine a second unit, the National Public Order Intelligence Unit, which operated from 1999 to 2011.

The inquiry has documented serious abuses by undercover officers, including deceiving women into sexual relationships and fathering children, spying on grieving families such as the parents of Stephen Lawrence, stealing the identities of dead children and lying in court. It has heard that one undercover officer, Mark Kennedy, infiltrated environmental and left-wing groups for six years and deceived a large, but unknown, number of women into intimate relationships. Evidence gathered by Kennedy was withheld from trials, leading to overturned convictions for some environmental campaigners. The probe is also scheduled to investigate an officer who used a false identity while infiltrating organized crime to form a 19-year relationship and father a child.

Campaigners say the Home Office consultation, which closes on Sunday, offers only three limited options: continuing with a narrower inquiry, replacing the inquiry with a process that would not have legal powers to compel testimony, or asking police to disclose documents to victims without any formal inquiry. More than 2,000 people have formally objected to the consultation, arguing that its practical effect would be to curtail the inquiry and conceal the full extent of wrongdoing.

The home secretary, Shabana Mahmood, has said she is concerned about the length of the inquiry and its cost to the taxpayer. The Home Office has framed the consultation as a way of asking those affected about the inquiry's future while also stressing a determination to secure justice and hold those responsible to account. Campaigners and victims say narrowing the remit or removing compulsion powers would foreclose lines of inquiry that have already revealed systemic failures in undercover policing.

The immediate next step is the closing of the consultation on Sunday, after which the Home Office will decide whether to propose changes to the inquiry’s structure or remit. Victims and campaign groups say any reduction in scope would leave central episodes of misconduct unexplained and justice unresolved.