Frances Crook, chief executive of the independent penal reform charity Howard League, has had two visits to privately-run prisons cancelled.

She published the letter from the National Offender Management Service (NOMS) on Twitter:

I have just received this pic.twitter.com/WxY0Qsn9Zb — Frances Crook (@francescrook) March 20, 2015

It acknowledges that she was invited by the security firm G4S to visit two of their prisons, but denies her access given her "comments about private prisons". Her charity is opposed to prisons being in private hands.

Outrage directed at the Ministry of Justice among a diverse range of politicians, including Labour shadow cabinet member Sadiq Khan, Lib Dem MP Julian Huppert and Ukip's Mark Reckless, ensued after Crook posted the letter online.

She also wrote a piece on the website politics.co.uk about her cancelled visits, with the headline: "I've been barred from visiting a G4S prison". In the piece, she writes that a G4S representative invited her to see Oakwood, a high-profile prison that has suffered much controversy since being taken over by the company, and Birmingham, the first Victorian prison to be handed over to the private sector.

I met a senior staffer from G4S in the studio of the BBC Radio 4 Today programme a few weeks ago when we were putting forward different views on privatisation and he invited me to visit Oakwood and Birmingham to see for myself. I was due to visit next week and had bought my train tickets. It was therefore a bit of a shock to get the letter from NOMS saying they were banning me. You know what they say, keep your friends close, and your enemies closer.

Crook received the letter on Friday this week, about an hour before putting it online. She tells me that her train tickets are booked to visit the two prisons on the same day: Monday 30 March. The plan was for her to be escorted for two to three hours around each prison by G4S staff at all times. She tells me she was invited three to four weeks ago, but only found that "NOMS was banning me from visiting" when she received the letter.

She is "still considering" her response to the Ministry, but tells me, "I hope they will reconsider."

She adds: "Members of parliament have got in touch with me and said they will take it up with the Minister for me. There are two on the Twitter feed: an interesting alliance, Julian Huppert from the Lib Dems, and Mark Reckless. And Sadiq Khan has also retweeted. There may well be others. It's right across the spectrum, people saying this is wrong."

Crook, who has visited almost every prison in the country during her 25 years working "in the system", was interested to see how the private sector had affected each of the prisons. Having visited Birmingham prior to the G4S takeover, she wanted to see "how it had changed".

Her organisation's view is that prisons should not be run by the private sector. "I know some of the private sector prisons actually do some quite good work, and I've always said that," she says. "That's not the point; they shouldn't be doing it. If you take away someone's freedom, it is the state's responsibility.

"So it's not that it's a better prison or a worse prison, the point is they shouldn't be doing it. And that's the distance between us. But we can have a grown-up discussion about that."

Regarding her cancelled visits, she says: "I go to prisons all the time. I entirely understand that prisons are places where people live and work, and you don't want to troop around out of prurient curiosity. We don't take groups around prisons to have a gawp. I think that's wrong.

"But I work in the system, have done for 25 years. It's really important if we're going to commentate and do research that we know what's going on. The critique from someone who's impartial is really important. Our independence is incredibly important and it can be very uncomfortable for government sometimes. But I think that's our job: speaking truth to power is our job."

Yet a spokesperson for the Prison Service warns that "those who irresponsibly misrepresent" the situation in prisons with "inaccurate comments" are unlikely to be first on the invitation list.

They commented:

Organisations and individuals independent of the Ministry of Justice and Prison Service are frequently given access to our prisons - Inspectors, Monitoring Boards, MPs, researchers and a wide range of interest and reform groups. It is absolutely right that prisons, like all public institutions, face significant media scrutiny and we welcome public debate on the issues they face. Groups and individuals are of course entitled to express their opinions. Those who irresponsibly misrepresent the situation by making inaccurate comments, and who fail to correct them when their inaccuracy is pointed out to them, are not a priority for access to prisons.

But Crooks insists on her expertise: "The problem with NOMS, and perhaps it goes to ministers - who knows? - is that actually I think they're being very immature, they're being petulant, and that's not a good way to behave. Take criticism on the chin. It's informed criticism. I know what I'm talking about, I know prisons."

UPDATE 11.38am 23/3/15

A blogger on legal matters, David Allen Green, has written a piece calling the MoJ's letter "disgraceful":