Boris Johnson has declared money spent on non-recent child abuse investigations as “spaffed up a wall”, prompting immediate criticism from Labour for making reckless and inappropriate comments.

The current favourite to succeed Theresa May as Conservative leader was arguing that police time and resources were being wasted on crimes committed years ago as he was questioned on an LBC radio phone-in on Wednesday morning.

He said: “And one comment I would make is I think an awful lot of money and an awful lot of police time now goes into these historic offences and all this malarkey.

“You know, £60m I saw was being spaffed up a wall on some investigation into historic child abuse and all this kind of thing. What on earth is that going to do to protect the public now?”

Louise Haigh,the shadow policing minister, said Johnson’s remarks were insulting to survivors of abuse.

“Could you look the victims in the eye and tell them investigating and bringing to justice those who abused them, as children, is a waste of money?” she asked.

Johnson’s remarks came hours after Cardinal George Pell was sentenced in Australia to six years in prison after being convicted of sexually abusing two 13-year-old choirboys in 1996. He is the most senior member of the Catholic clergy to be convicted of child abuse offences. Pell’s appeal is to be heard in June.

Ministers set up the independent inquiry into child sexual abuse after the Jimmy Savile revelations prompted reporting from around the country of a string of abuse scandals that in many cases had been covered up or not investigated for years.

The inquiry, chaired by Prof Alexis Jay, is in the middle of a three-week examination of claims involving politicians. It heard earlier this week that Eliza Manningham-Buller, a former director general of MI5, stayed away from the 1995 funeral of Peter Morrison, a Conservative minister under Margaret Thatcher, over allegations of child sex abuse and his pretence that she was his girlfriend. Morrison had denied the allegations.

An ally of Johnson said he had no intention of apologising or clarifying his remarks about child abuse cases. The MP was making the point that spending tens of millions on historic cases where an alleged perpetrator was dead should not be the priority when the cash could be used on front line policing and tackling knife crime.

Johnson consistently leads polls of Conservative party members when they are asked who they would like to be the next leader. The former foreign secretary, who resigned from the government last summer, voted against Theresa May’s Brexit deal on Tuesday.

He told MPs the prime minister had only secured minimal improvements to her Brexit agreement, with “the result that, like Adam and Eve in the Garden of Eden, they have sewed an apron of fig leaves that does nothing to conceal the embarrassment and indignity of the UK”.