Keir Starmer has refused to back the archbishop of Canterbury, who has faced growing demands to resign over his handling of an abuse scandal.

Pressure on Justin Welby has been intensifying since the publication last week of a damning report on the church’s cover-up of John Smyth’s abuse in the UK in the late 1970s and early 1980s, and later in Zimbabwe and South Africa. About 130 boys are believed to have been victims.

The independent review into the abuse concluded that Smyth may have been brought to justice had the archbishop formally reported it to police a decade ago.

The prime minister said on Tuesday that Smyth’s victims had been “failed very, very badly”. He would not comment directly on whether Welby should quit.

Speaking to reporters from the Cop29 summit in Baku, Azerbaijan, Starmer said: “Let me be clear: of what I know of the allegations, they are clearly horrific in relation to this particular case, both in their scale and their content. My thoughts, as they are in all of these issues, are with the victims here who have obviously been failed very, very badly.

“It’s a matter, in the end, for the church, but I’m not going to shy away from the fact of saying that these are horrific allegations and that my thoughts are with the victims in relation to it.”

His comments came as Welby faced calls to resign from Smyth’s victims and the clergy, including the bishop of Newcastle.

One victim, Andrew Morse, who encountered Smyth while a pupil at Winchester College, Hampshire, said Welby should resign in solidarity with abuse victims. He told BBC Radio 4’s Today programme on Tuesday that Welby’s admission he had not done enough since 2013 “is enough in my mind to confirm that Justin Welby along with countless other Anglican church members were part of a cover-up about the abuse”.

Stephen Cherry, the dean of chapel at King’s College, Cambridge and a former canon of Durham Cathedral, told the BBC: “I think he really needs to now tender his resignation and allow there to be significant change.”

On Monday, Helen-Ann Hartley, the bishop of Newcastle, said Welby’s position was untenable and he should quit and that a line needed to be drawn.

Welby said last week he had considered resigning over his “shameful” decision not to act on reports of abuse by Smyth, a powerful and charismatic barrister who abused private schoolboys at evangelical Christian holiday camps, when he was told about them in 2013. Smyth died in 2018.

Lambeth Palace said on Monday that Welby had “apologised profoundly both for his own failures and omissions, and for the wickedness, concealment and abuse by the church more widely” but the archbishop did not intend to resign.