The family of the man whose shooting by police triggered last summer's riots has condemned the watchdog investigating the pre-planned operation for withholding details that could scupper a full inquest.

The family's solicitor said its confidence in the Independent Police Complaints Commission (IPCC) was disappearing after learning that material about police decision-making on the day Mark Duggan was shot could not be provided to the coroner in charge of his inquest.

The coroner yesterday put back the planned opening of the hearing until next year and will rule in October if the court has enough information for it to go ahead. A special inquiry could be held instead, where some evidence is heard behind closed doors.

"This is the latest twist in the Duggan investigation and leaves them [the family] uninspired by the IPCC's assertion of independence," the family's solicitor, Marcia Willis-Stewart, said. "I don't know about public confidence, but the family's confidence is going by the day."

The delay marks the latest conflict between the family and the watchdog investigating the circumstances of the shooting. The IPCC said it was expecting to deliver its final report into the incident to the coroner by early autumn – more than a year after Mr Duggan's death.

But it added that it may have material "it could not properly disclose to a coroner" over the decision by officers to shoot Mr Duggan, North London Coroners' Court was told yesterday.

The IPCC said it had informed the family about the change of circumstances, but the family said it was concerned that the organisation was acting as a "shield" for the police.

Mr Duggan was a passenger in a minicab when he was killed by a single shot to the chest during an operation involving officers from Scotland Yard's Trident gun crime unit on 4 August in Tottenham, north London. Anger over the shooting led to riots in the area, with unrest soon spreading across London and then to other parts of the country.

The police and the IPCC have apologised to Mr Duggan's parents for failing to tell them directly about their son's death. They only learned of the killing from watching television, according to their MP, David Lammy.

Initial reports suggesting that Mr Duggan shot at police were later dismissed by ballistics tests, which found that a bullet lodged in one officer's radio was issued by police. The inquiry failed to establish the sequence of events concerning a handgun inside a box that was reportedly found at the scene of the shooting. The IPCC said yesterday that a criminal trial of two men about the circumstances in which Mr Duggan allegedly acquired the gun would run into October and was the reason for the inquest's delay.

"The IPCC also alerted the coroner and other interested persons to the possibility that it is likely to be in possession of material that would be relevant to the issue of police decision-making but which could not be provided to the inquest for legal reasons," it added in a statement.