Last summer, as a storm over phone hacking raged around Rebekah Brooks, a friend texted her with consoling advice. Not long afterwards, she resigned from her job as the chief executive of News International and was almost immediately arrested and bailed by detectives investigating allegations of phone hacking. The friend who texted, according to a new biography by James Hanning and Francis Elliott, was David Cameron. The authors say that Brooks's husband Charlie told friends about the message, which urged her to "keep her head up and she'd get through her difficulties". That doesn't say much for Cameron's judgement, but it's an insight into the close relationship with Brooks.

Their friendship will come under the spotlight tomorrow when Brooks appears before the Leveson Inquiry. In an eventful week, the inquiry heard yesterday that the News of the World did hack Milly Dowler's phone, but the truth about voicemail deletions might never be known; and Cameron's former communications chief, Andy Coulson, is due to give evidence today.

Last week, Cameron was one of eight Cabinet ministers who made a late application to become "core participants" in the inquiry, allowing them to see key witness statements in advance. After Jeremy Hunt's mauling last month, when the Culture Secretary had no notice of emails handed to the inquiry by James Murdoch, I can see why they were keen to acquire this status. But there's an important question about which parts of their involvement can be funded at public expense.

Last week's application was made by James Eadie QC; as First Treasury Counsel, he has appeared for ministers on government business such as the proposed extradition of Abu Hamza. Yet many of the questions that Cameron faces are about a personal friendship which pre-dates his period in government. And other ministers are likely to be asked about events, such as Hunt's visit to News Corp in 2009, which took place while in Opposition.

I am also a core participant, along with other victims of phone hacking, and we've had to find private sources of funding for legal representation. Yesterday, a Cabinet Office spokesperson told me that "Government is providing legal support for ministers for the parts of their evidence that relate to government business". I'm not sure the distinction is clear, but the Cabinet Office confirmed that it is up to ministers to arrange their own legal representation for evidence relating to the period when they were in Opposition.

Shortly after last summer's text message, Cameron cooled towards Brooks. She didn't want to embarrass him, according to Hanning and Elliott, and he wanted to be able to say they hadn't been in touch. Embarrassment now seems inevitable, along with the intriguing prospect of Cabinet ministers having to urgently consult lawyers about what is and isn't "government business".