Boris Johnson has been lambasted for his use of "crass language" about historic child sex abuse cases.

The former foreign secretary said money spent investigating cases was a "malarkey".

His comments came as he spoke about spending public money on police resources to tackle knife crime during an interview on LBC.

"Keeping [police] numbers high on the streets is certainly important, but the question is where you spend the money and where you deploy the officers," the former mayor of London said.

"One comment I would make is I think an awful lots of money, an awful lot of police time now goes into these historic offences and all this malarkey.

"£60m I saw was being spaffed up the 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?

"What the people want is to see officers out on the streets doing what they were signed up to do."

But the NSPCC said his comments were offensive to victims of abuse.

A spokesperson said: “Bringing child abuse perpetrators to justice is not a ‘malarkey’ and such crass language is an affront to victims who have suffered in silence for decades.

“Investigation of historic allegations is an important part of creating a culture where we can talk more openly about abuse.

“These investigations will also help us all to learn lessons so that as a society we do not repeat the mistakes of the past.”