The Church of England is reviewing a complaint against the incoming archbishop of Canterbury over her handling of an abuse allegation.

Dame Sarah Mullally is due to take up the role next month, after Justin Welby was forced to resign over the way he dealt with a safeguarding scandal.

Mullally has been has been accused of mishandling a complaint against a priest in London, where she serves as bishop.

The alleged victim, referred to only as N, said the abuse started in 2014 and was reported to the Diocese of London. Mullally became Bishop of London in 2018.

N claimed that after he made a formal complaint about the abuse in 2019, Mullally breached a church disciplinary code by sending a confidential email about the allegations to the priest concerned, according to Premier Christian News, which first reported the claims.

N told the news site that the Diocese of London’s and Mullally’s handling of the complaint had left him feeling suicidal.

The Diocese of London said proper processes had been followed and that there was no outstanding complaint against Mullally.

Officials at Lambeth Palace, the official London residence of the archbishop of Canterbury, said a complaint about Mullally’s handling of the allegation had been made in 2020 but had not been followed up on due to “administrative errors and an incorrect assumption about the individual’s wishes”.

Mullally was not informed of the complaint at the time, church authorities said.

A Lambeth Palace spokesperson said church authorities had written to N to outline the next steps.

“Due to administrative errors and an incorrect assumption about the individual’s wishes, the complaint was not taken forward or appropriately followed up,” they said.

“The Bishop of London was unaware of the matter, as the process never reached the stage at which she would have been informed of the complaint or its contents.

“The provincial registrar has apologised to those involved and urgent arrangements are now being made for the complaint to be considered according to the relevant statutory process.”

The complaint will be assessed and sent to Stephen Cottrell, the archbishop of York, who faced calls to resign last year over his handling of a separate sexual abuse case and is now responsible for deciding whether the complaint should be dismissed or warrants a formal response.

Cottrell has the authority to refer the matter to a conciliator, impose sanctions or escalate it to a church tribunal for further investigation.

Mullally said the alleged victim had been “let down” and that she was seeking assurances procedures had been reformed.

She added: “While his abuse allegations against a member of clergy were fully dealt with by the Diocese of London, it is clear that a different complaint he subsequently made against me personally in 2020 was not properly dealt with.

“I am seeking assurance that processes have been strengthened to ensure any complaint that comes into Lambeth Palace is responded to in a timely and satisfactory manner.

“The church’s processes have to change, both for complainants and for the clergy who are the subject of complaints. Today, I am one of those clergy.

“As archbishop of Canterbury, I will do everything in my power to bring about much needed and overdue reform. We must have trust in our systems, or else we cannot expect others to put their trust in us.”

Welby’s resignation followed an independent review by Keith Makin, which concluded Welby had not done enough to address allegations of abuse by the Christian camp leader John Smyth.

The report stated that Smyth “could and should have been formally reported to the police in the UK, and to authorities in South Africa (church authorities and potentially the police) by church officers, including a diocesan bishop and Justin Welby in 2013”.