Welby, the senior bishop of the Church of England and spiritual leader of 85 million Anglicans worldwide, had faced calls to resign after a report last week concluded he had taken insufficient action to stop a person it described as arguably the Church's most prolific serial abuser.
In his resignation letter, Welby said he must take "personal and institutional responsibility" for lack of action on the "heinous abuses".
"The last few days have renewed my long felt and profound sense of shame at the historic safeguarding failures of the Church of England," Welby said in a statement.
"I hope this decision makes clear how seriously the Church of England understands the need for change and our profound commitment to creating a safer church. As I step down I do so in sorrow with all victims and survivors of abuse."
Statement from the Archbishop of Canterbury.— Archbishop of Canterbury (@JustinWelby) https://t.co/aNnuLBMapo pic.twitter.com/pIIR1911QUNovember 12, 2024
Archbishop of York Stephen Cottrell, the Church's second-ranking cleric, called Welby's resignation "the right and honourable thing to do".
Welby's tenure covered a decade of major upheaval in which he was forced to navigate rows over homosexual rights and women clerics between liberal churches, mostly in North America and the United Kingdom, and their conservative counterparts, especially in Africa.
The Anglican churches in African countries such as Uganda and Nigeria are likely to welcome Welby's resignation, after saying last year they no longer had confidence in him.
His successor's main challenges will include holding together the increasingly fractious worldwide Anglican community and attempting to reverse a decline in church attendance, which is down a fifth in the UK since 2019.
Welby resigned five days after the independent Makin Report singled him out for criticism over his handling of abuse allegations dating back to the 1970s.
The report said John Smyth, a UK lawyer, had subjected more than 100 boys and young men to "brutal and horrific" physical and sexual abuse over a 40-year period.
Smyth beat some victims with up to 800 strokes of a cane and supplied nappies to absorb the bleeding, the report said.
He would then drape himself over his victims, sometimes kissing them on the neck or back.
Smyth was chair of the Iwerne Trust, which funded the Christian camps in Dorset in England, and Welby worked at them as a dormitory officer before he was ordained.
Smyth moved to Africa in 1984 and continued to carry out the abuse until close to his death in 2018, the report said.
The Church of England knew at the highest level about the sexual abuse claims at the camps in 2013 and Welby became aware, at the latest, about the accusations in the same year, months after he became archbishop, according to the report.
If the claims had been reported to the police in 2013, there could have been a full investigation and Smyth might have faced charges before he died, the report said.
The Makin Report was commissioned in 2019.
Welby apologised for "failures and omissions" but said he had "no idea or suspicion" of the allegations before 2013.
The report concluded this was unlikely, accusing him of failing in his "personal and moral responsibility" to ensure a proper investigation.
Church procedures for the appointment of a new archbishop of Canterbury require a body of clerics and a chair, nominated by the UK prime minister, to put two names forward to him.
Graham Usher, the Bishop of Norwich, and Guli Francis-Dehqani, the Bishop of Chelmsford, have both been tipped to succeed Welby and become the 106th Archbishop of Canterbury.
Usher is in favour of gay rights and has been outspoken on the need to tackle climate change.
Francis-Dehqani was born in Iran and has spoken about how her brother was murdered in the aftermath of the Iranian revolution.
She would be the first woman to occupy the post.
