The police regulator says it has been tracking down witnesses across the world, as it seeks to explain why it has yet to decide whether to launch an investigation into Boris Johnson and his relationship with the US businesswoman Jennifer Arcuri.

Since late September the Independent Office for Police Conduct (IOPC) has been evaluating whether to investigate the prime minister for possible criminal misconduct over his friendship with Arcuri when he was mayor of London.

Its decision had initially been expected in November, until the Observer learned that an announcement would be delayed until after the general election in December, effectively protecting Johnson from potentially damaging headlines at a crucial stage of the campaign.

Now London assembly members fear that the continuing and “surprising” delay by IOPC investigators could mean that no announcement will be forthcoming before the period of “purdah” that begins next month before the London mayoral elections.

They say they were initially told that the IOPC investigation into Johnson – who, when he was mayor, had responsibility for policing in London – would be “done and dusted” by the end of November, allowing the GLA’s oversight committee to resume its own investigation into Johnson’s relationship with Arcuri.

IOPC sources have confirmed that the watchdog is still engaged on a “scoping exercise” into the allegations against the prime minister and said that the body had “never put a timescale on how long this process will take”. It has previously denied that a decision was delayed to spare Johnson potential embarrassment before the election, and insists that the information-gathering exercise into the claims has been complex.

“The allegations date back around eight years, and any potential evidence which would help us to decide whether or not those reasonable grounds exist is information we have to access from other organisations. We have also had to locate and speak with a number of potential witnesses both in this country and abroad,” the IOPC said in a statement.

Confirmation that the investigation has widened internationally is potentially significant. Among the questions that have arisen over Johnson and Arcuri, a technology entrepreneur, include how she secured a place on three City Hall foreign trade missions when he was mayor. Reports have indicated these encompassed a November 2015 trip to Tel Aviv, a mayoral trip to Malaysia and Singapore in November 2014, and one to New York in February 2015.

Confirmation that the police watchdog is still deliberating over its decision after almost 150 days has prompted dismay from London assembly members.

Caroline Pidgeon, a Liberal Democrat member of the London assembly’s oversight committee, said: “It beggars belief that after all these months the IOPC is still looking at whether or not to investigate the former mayor of London. What is taking so long? It makes no sense and is simply delaying the work of the London assembly looking into this matter and the use of City Hall resources.”

Len Duvall, Labour chair of the oversight committee, said : “I am surprised it is taking this long and I’ve got a feeling it’s just being dragged out now ahead of purdah. It just stops us doing our investigation.”

Among the allegations being investigated by the IOPC are questions over a payment of £126,000 of public money to Arcuri’s businesses and why she was allowed on three overseas trade missions with Johnson despite not qualifying as a delegate.

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