Boris Johnson will find out on Thursday if he faces a criminal investigation into his relationship with an American businesswoman while he was mayor of London. The Independent Office for Police Conduct (IOPC) is to reveal its long-delayed decision on whether to investigate him for possible criminal misconduct over his friendship with Jennifer Arcuri. The watchdog will make the announcement at noon, after spending months on a scoping exercise into whether the prime minister has a case to answer. Johnson was formally referred to the watchdog in September because he was 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 when he was mayor. Arcuri was awarded thousands of pounds in public money, including £11,500 by the mayor’s promotional agency, London & Partners. Johnson’s office also intervened to give her a place on trade missions to New York and Tel Aviv with Johnson, after she was initially turned down because she failed to meet the criteria. The IOPC has repeatedly insisted that it has received no pressure from the government over the decision, which had been expected before last December’s general election, or over its timing. The watchdog has a duty to investigate any alleged criminal offence committed by a figure in charge of policing, whether or not they are still in office. That assessment has now concluded almost eight months after it was first referred to the IOPC. Johnson has insisted that he acted with “full propriety” and claimed he had no interest to declare in Arcuri, who has said she was given no favours by Johnson. Arcuri has conceded, however, that the then mayor should have declared their friendship. In response to a freedom of information request, the IOPC said earlier this month that it was still conducting its scoping exercise. In a letter to the social policy and transparency thinktank Society Syndrome, it said: “We are aware of the public interest in this matter and once a decision has been made, we will make a further statement.” It has strenuously denied delaying the decision to spare the prime minister potential embarrassment before the last election. The watchdog is understood to have reached its decision believing that whichever way it goes, it may face a legal challenge, such as a judicial review, from those who disagree with it. If it decides against a criminal investigation, it will not be the end of the matter. A special committee of the Greater London Authority will pick up the allegations, its chair told the Guardian. Len Duvall, chair of the oversight committee and a Labour member of the London assembly, said: “We’ve been waiting for the IOPC to finish. If they come back with a negative conclusion [not to investigate], it comes back to the oversight committee. We will look at the evidence and come to a conclusion.” The oversight committee works to a lower standard of proof than the “beyond reasonable doubt” threshold required for criminal action, but cannot apply any sanction other than a public report. Earlier this year, critics accused the IOPC of dragging its feet over the decision. Siân Berry, the Green party co-leader who is standing as the Greens’ candidate in next year’s mayoral election, said in January: “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.” Siobhan Benita, the Liberal Democrat candidate for London mayor, said: “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.”