The Prime Minister's wife Cherie Blair asked for a media blackout when she addressed students at the historic Oxford Union on Friday night.
Mrs Blair, whose professional name is Cherie Booth, was speaking in her capacity as a human rights QC for the legal firm Matrix Chambers.
But according to the Oxford Union president Theo Roos, Mrs Blair had decided to be coy about journalists wishing to attend the event.
He said: "It would be great for the Oxford Union to be able to open this to the press and great publicity for us.
"The only problem we have is that she would not speak if there were press there. She only wants Oxford Union members."
He said that speakers at the Oxford Union, who have in the past included presidents, pop stars and actors of screen and stage, were always given the option of speaking without the press in attendance.
"It's not something that we wanted to have to in force but obviously our first priority is to get speakers."
A spokesman for No 10 Downing Street said they would not speak about any of the Prime Minister or his wife's engagements in advance.
A spokesman for Cherie Blair's Matrix Chambers said it could not comment on matters that did not concern Mrs Blair's work for the chambers.
A former Oxford University press officer, who did not wish to be named, said that speakers did often request private sessions. These are understood to have included the footballer John Terry and former Army Chief of Staff General Sir Mike Jackson.
She said: "They are all given the option of having no press. I don't think it's particularly unusual or particularly newsworthy that Cherie Blair has chosen not to, it just means that people can speak with freedom without feeling that they are going to be quoted."