A Church of England bishop has called on the Archbishop of Canterbury to resign, calling his position “untenable” after a damning report into a prolific child abuser associated with the Church.

Bishop of Newcastle Helen-Ann Hartley is the most senior member of the Church to call on the Most Reverend Justin Welby to stand down, following the “horrific, horrendous and shocking” report.

Mr Welby is facing mounting pressure to resign after it emerged last week that he did not follow up rigorously enough on reports of John Smyth QC’s “abhorrent” abuse of more than 100 boys and young men.

A review of the Church’s handling of Smyth’s case said the archbishop “could and should” have reported the case to authorities when details were presented to him in 2013.

Mr Welby acknowledged he should have more rigorously followed up the details and said last week he had considered resigning, but decided to stay in his role.

The Makin review into Smyth’s case said he might have been brought to justice for decades of abuse before his death, in 2018, had he been formally reported to the police in 2013.

Smyth is believed to be the most prolific serial abuser to be associated with the Church of England, having subjected as many as 130 victims to traumatic physical, sexual, psychological and spiritual attacks.

Smyth’s abuse took place over almost five decades and across three countries, according to the report. He targeted boys who attended summer camps he ran for young Christians.

Smyth abused 26 to 30 boys and young men in the UK in the 1970s and 1980s, the report found. He then relocated to Africa, where he abused a further 85 to 100 “young male children aged 13 to 17”.

The report says that from July 2013, the Church of England knew “at the highest level” about Smyth’s abuse in the UK and should have “properly and effectively” reported him to the UK police and the relevant authorities in South Africa.

Inaction from the Church represented a “missed opportunity to bring him to justice,” the report says.

Hampshire Police opened an investigation into Smyth after a Channel 4 documentary brought allegations against him to light in 2017.

Shortly after it aired, Mr Welby told Channel 4: “I genuinely had no idea that there was anything as horrific as this going on and the kind of story you showed on the clip.

“If I’d known that, I would have been very active, but I had no suspicions at all.”

But last week’s report said “enough was known to have raised concerns upon being informed in 2013”.

Andy Morse, one of Smyth’s victims, told the Telegraph: “I don’t believe he was telling the truth.

“I’m not sure that he would have had knowledge of the detail, but I think he would have had knowledge of the summary.”

Anglican priest Giles Fraser told the BBC Mr Welby had “lost the confidence of his clergy” and three members of the Church’s parliament – the General Synod – have started a petition calling for the archbishop to resign over his “failures” to report Smyth’s abuse.

Asked whether Mr Welby should stand down, Prime Minister Sir Keir Starmer said: “That’s a matter really for the Church rather than for me.”

“It’s very hard to find the words to respond adequately to what the report tells us,” Bishop Hartley told the BBC.

“I think rightly people are asking the question ‘Can we really trust the Church of England to keep us safe?’ And I think the answer at the moment is ‘no’,” she said.

Dr Hartley said it would be hard for the Church to “continue to have a moral voice” when “we cannot get our own house in order with regard to something as critically important”.

“We are in danger of losing complete credibility on that front,” she added.

She said Mr Welby’s resignation would not “solve the safeguarding problem,” but it would “be a very clear indication that a line has been drawn, and that we must move towards independence of safeguarding”.

Smyth was accused of attacking boys at his home in Winchester in the 1970s and 1980s, identifying them at Christian camps he ran and at leading public schools including Winchester College.

Smyth took them to his home where he carried out lashings with a garden cane in his shed.

One of Smyth’s victims, Bishop of Guildford Andrew Watson, previously described the “excruciating and shocking” abuse he experienced.

A report detailing Smyth’s abuse was presented to some Church leaders in 1982, but no report was made to the police.

He was encouraged to leave the country and moved to Zimbabwe and later to South Africa, where his abuse continued in the years leading to his death in 2018.

Smyth was charged with the manslaughter of a 16-year-old boy at one of his summer camps. He was not convicted of the offence.

Smyth died aged 75 while under investigation by Hampshire Police.

In a statement, Mr Welby said he was “deeply sorry that this abuse happened” and “sorry that concealment by many people who were fully aware of the abuse over many years meant that John Smyth was able to abuse overseas and died before he ever faced justice”.

He added: “I had no idea or suspicion of this abuse before 2013.”

The petition calling for Mr Welby’s resignation was created on Saturday by three of the General Synod’s almost 500 members, and is open to anyone to sign online. It had gathered over 5,600 signatures by Monday evening.

“Given his role in allowing abuse to continue, we believe that his continuing as the Archbishop of Canterbury is no longer tenable,” the petition reads.

“We must see change, for the sake of survivors, for the protection of the vulnerable, and for the good of the Church.”

Mr Fraser, vicar of St Anne’s Church at Kew, west London, told BBC Radio 4’s Today programme Mr Welby “really [had] to go”.

Recalling his own experience of abuse at school, Mr Fraser said such an experience was “very traumatic and stays with you”.

“This happened to me when I was seven, eight – I’m 60 in a few weeks’ time,” he said.

“The idea that people continued to be abused after the Church knew what was happening is disgraceful.”

At the weekend, the Church’s lead safeguarding bishop said she welcomed Mr Welby’s apology – and would not say whether he should resign.

“I really appreciate that the Archbishop has wholeheartedly apologised for what he could have and should have done differently in 2013,” the Rt Rev Joanne Grenfell, the Bishop of Stepney, told the BBC.

“I also recognise his commitment over the time of his tenure as Archbishop to really having tried to change safeguarding.”

A spokesperson for Mr Welby said the archbishop hoped the Makin review would support the ongoing work of building a safer Church in the UK and around the world, and reiterated his “horror at the scale of John Smyth’s egregious abuse, as reflected in his public apology”.