Labour will look "favourably" on requests for an investigation into sex in prison if it wins the next election, Sadiq Khan has said.
Writing for Politics.co.uk, the shadow justice secretary said Chris Grayling was "burying his head in the sand" when he blocked an inquiry into sex and rape behind bars by the Howard League for Penal Reform.
"Sexual violence and rape does go on, and we should be determined to root it out and do all we can to stop it. Crimes are crimes even when behind bars," Khan wrote.
"Instead, Chris Grayling wants to bury his head in the sand and ignore it, probably because it doesn't fit his right-wing agenda of throwing red meat to his restless backbenchers."
Reports that Grayling blocked the investigation into sex in prison outraged penal reform campaigners and public health experts, some of whom believe there could be an unaddressed epidemic of sexual assaults behind bars.
In the US, authorities insisted for years that rape in prison was not a problem until an investigation revealed abuse on a previously unimagined scale. The investigation led to the Prison Rape Elimination Act last year.
Some believe Grayling's refusal to allow the Howard League investigation stems from his personal animosity towards the pressure group.
"This government appear to have declared war on civil society," Khan wrote.
"Stung by criticisms of their policy, they've pursued an agenda designed to silence those daring to question the impacts of government legislation.
"Politics.co.uk's report that Chris Grayling has blocked research into sex in prisons by the Howard League for Penal Reform fits this pattern of behaviour.
"Some suggest it's because they've been a persistent thorn in his side."
He added: "I've been on the end of the odd stinging rebuke from the Howard League myself, but that wouldn't mean I'd want to shut them up. What is the point of having pressure groups if they don't put pressure on politicians?
"Should I become justice secretary, a request to research the subject of sex in prisons would be looked on favourably. After all, this is isn't a frivolous topic, like some kind of 1970s Carry On film. It's deadly serious."
The Howard League investigation into sex in prison has been forced to rely on evidence sessions with former prisoners, after Grayling barred it from talking to any inmate currently doing time or out on licence.
Anecdotal reports suggest coercive sex may be rife in prison, with victims who do lodge complaints being mostly ignored by prison staff and attacked by other prisoners for reporting on fellow inmates.