The police watchdog is facing accusations that it is dragging its feet on a decision about whether to investigate Boris Johnson for possible criminal misconduct over his friendship with the US businesswoman Jennifer Arcuri when he was London mayor.

The Independent Office for Police Conduct (IOPC) insists it has received no pressure from the government over the decision, which had been expected before last month’s general election, or its timing.

In October the IOPC indicated it would take about a month to decide whether Johnson had a case to answer. But the watchdog now says it is still conducting a “scoping exercise” on the decision. It has also continued to request new information from the mayor’s promotion agency, London and Partners (L&P), this month.

Boris Johnson was formally referred to the watchdog because he was also the head of the Mayor’s Office for Policing and Crime at a time when Arcuri received public money and access to trade trips led by Johnson via L&P.

Johnson has insisted he acted with “full propriety” and claimed he had no interest to declare. Arcuri has said she was given no favours by Johnson but she has conceded that the then mayor should have declared their friendship.

In response to a freedom of information [FoI] request, the IOPC said: “It is not possible to confirm a precise date when the scoping exercise will be completed as relevant information and advice is still being considered.”

It previously strenuously denied delaying the decision to spare the prime minister potential embarrassment before the election.

In a letter to the social policy and transparency thinktank Society Syndrome, which submitted the FoI request, Sal Naseem, the IOPC London director, said: “The IOPC has not decided to delay the decision … our assessment is ongoing as we continue to collect information and seek legal advice.”

Asked if the government had tried to influence the decision or its timing, Naseem said: “No.”

Michael Desmond, the chair of the thinktank, said he could not understand the reason for the delay. He said: “How long does it take to do that when it is quite evident what has happened and how it came about because it is in the public domain? They don’t need to drag their heels any longer.

“I don’t want Boris to be put in prison for life, which is the maximum sentence for this, but I do think he should be held to account.”

Two London mayoral candidates have also expressed frustration at the time IOPC is taking. Siân Berry, the Green party co-leader who is standing as the Greens’ candidate in May’s mayoral election, said: “This has gone on too long.”

She added: “They should have made a decision before the general election. When there are big issues about someone’s conduct in public life and they are up for election for the most senior post in the government, there’s no real justification for delay. The public should have been able to see all the information that they needed to make a decision. We didn’t expect it to drag on – justice delayed is justice denied.”

Siobhan Benita, the Liberal Democrat candidate for mayor, said: “I’m very concerned the IOPC still hasn’t made a decision. It’s already outrageous this whole episode was effectively sat on during the general election when there was sufficient evidence for an investigation to proceed. I’m beginning to wonder if we will ever find out what really happened between the pair.

“It’s imperative that the prime minister isn’t allowed to wriggle out of giving evidence and that the London assembly is able to do its job properly.”

In his letter, Naseem said the agency was subject to election purdah rules, under which policy announcements are restricted in the run-up to an election. But he suggested this was not the reason for the delay.

“The decision and its announcement will be made when all relevant information and advice has been considered,” Naseem said.