IPCC says now is not the time to pass judgment on shooting, which was prelude to rioting across England

Two members of a group set up to ensure community confidence in the investigation of the police shooting of Mark Duggan have resigned, the Guardian has learned.

The three-member community reference group was set up by the Independent Police Complaints Commission after Duggan was shot dead by a Scotland Yard marksman on 4 August in Tottenham, north London. The shooting was the prelude to some of the worst rioting in modern British history.

One member of the group claimed the police watchdog is, in some ways, worse than the force it is investigating. He also raised concerns about the handling of key evidence at the scene of Duggan's death, claiming police were allowed to move it.

The IPCC appointed the group at the start of its investigation. The two members who have resigned, saying their confidence in the police watchdog's inquiry was damaged, are community activists Stafford Scott and John Noblemunn. The third member, Pastor Nims Obunge, has not resigned.

In a piece for the Guardian's comment pages, Scott says IPCC errors led him to lose faith in its investigation.

The three members of the community reference group were briefed on developments in the investigation and had to sign confidentiality agreements.

Duggan was followed by police as he travelled in a taxi and obtained a weapon. He was stopped at gunpoint, and later shot twice. The IPCC is investigating the circumstances.

In his article, Scott says the taxi Duggan had been travelling in was taken away from the scene by police just after the shooting, despite it containing "evidence of major significance".

Scott writes that Rachel Cerfontyne, the IPCC commissioner in charge of the Duggan investigation, was unaware of this. "Weeks after these revelations we were told by a very embarrassed IPCC commissioner that she had now been informed by her investigators that they had in fact sanctioned the removal of the vehicle. This was before they had even got to the crime scene.

"It took three months for this to be revealed to the [community reference group] and to the commissioner."

Confidence was shaken at the start, when the IPCC said Duggan had shot at police, only for ballistic tests to disprove this.

Scott said: "I have been alarmed to learn that not only have the IPCC broken their own guidelines by giving out erroneous information to journalists regarding the 'shoot-out' involving Mark Duggan and police that didn't actually happen. But I have discovered that their investigation … is flawed and in all probability tainted to a degree that means we will never be able to have faith in their final report into the killing."

Scott, who has also advised the Met, says the police watchdog suffers worse flaws than the force it is investigating. "In all of my years of engaging with the Met, I have yet to witness such a shoddy investigation," he said.

On 7 August, Cerfontyne explained the importance of the community reference group to the inquiry: "My role is to oversee the investigation – which must also support the family, and address the concerns expressed by the community. To help me, I have established a community reference group to ensure I am sensitive and responsive to them."

In a statement on Sunday night, she said: "During the early stages of a complex investigation emerging evidence can be confusing and is certainly incomplete. The time to pass judgment on our investigation is when it is complete and the full evidence will be publicly tested. As the commissioner I have every confidence that this investigation is thorough, robust and independent."

On Friday, revelations that police may have missed a chance to seize the gun Duggan subsequently acquired, days before he obtained it, led to raised tensions. Police put in place extra public order contingencies after it was revealed that the weapon may have been used in an assault reported to the police.

Early Monday morning Scotland Yard said it had charged a man with the murder of Kelvin Easton, reported to be a close friend of Duggan. After Duggan's shooting, some claimed Duggan had been buying a weapon to exact revenge, while others doubt that. Police said that Warren Allen had been charged with Easton's murder at an east London nightclub, the Boheme, in March.

The Metropolitan police said on Saturday that it would report the Guardian to the Press Complaints Commission. It said a headline for a Guardian article raising new questions over the shooting was misleading and inaccurate.

In a statement, the Met said it had reported to the PCC "… its concerns about a headline in the Guardian newspaper which stated: 'Man whose shooting triggered riots was not armed'.

"The headline was inaccurate and misleading based on what is known at this time and despite making this point in the strongest terms to the newspaper, it was not amended.

"The article, but particularly its headline, stated as fact issues that are subject to an ongoing investigation by the IPCC, who have themselves issued a statement expressing similar concerns. We would reiterate that it is premature for anyone to draw conclusions prior to completion of the IPCC's investigation."

The IPCC said on Saturday that it was also considering a complaint to the press watchdog and that its investigation still has to establish the sequence of events concerning a handgun found at the scene.

The IPCC said: "This is a complex investigation that involves gathering information including witness statements, pathology, forensics and ballistics analysis and we have stated to the coroner that it will be completed within four to six months," the statement said.

"One of the key elements we will seek to establish is the sequence of events concerning the non-police-issue firearm found at the scene.

"That has not been established yet, contrary to what has been written in the Guardian article today.

"We would urge people not to rush to judgment until our investigation is complete and they have the opportunity to see and hear the full evidence themselves."

The article itself said the investigation has found no forensic evidence that Duggan was carrying a gun when he was shot, and that a gun collected by him earlier in the day was found 10 to 14 feet away from his body, on the other side of a low fence.

The headline was amended online on Saturday pending further investigation of the complaint.The Sunday Times said it had been told by three sources that the gun was inside a shoebox in the minicab in which Duggan was traveling. The paper reported that police sources claimed that when Duggan was challenged by officers, Duggan made to pull the gun on them.