Rupert Murdoch's UK newspaper chief told David Cameron the night before a crucial political speech in 2009 that they were "professionally in this together", an inquiry revealed yesterday, embarrassing the man who now rules Britain.

The text message to Cameron, then in opposition, from Rebekah Brooks, then the head of News Corp's British newspaper operation, was read out to the prime minister on live television during a grilling about his ties to Murdoch's media empire.

"I am so rooting for you tomorrow not just as a personal friend but because professionally we're definitely in this together. Speech of your life? Yes he Cam!" Brooks told Cameron in an SMS the night before his party conference speech.

Testifying under oath at the Leveson inquiry into media ethics, Cameron said Brooks had merely meant that they had a common interest because her Sun newspaper had come out in support of his Conservative Party ahead of the 2010 election.

But the message makes excruciating reading for Cameron as ''We're all in this together" was the Conservatives' campaign slogan for that election, while "Yes he Cam" was The Sun's headline the day after he made the 2009 conference speech.

The message also shows how close Cameron once was to Brooks, who quit as head of Murdoch's British newspaper arm last year over phone hacking by reporters and has since been charged with perverting the course of justice for allegedly hiding evidence.

Cameron ordered the Leveson inquiry last year at a time when he was under pressure to crack down on the Murdoch press because of the revelation that reporters at the News of the World tabloid had hacked into the phone of a murdered schoolgirl.

But if Cameron had hoped the inquiry might take some heat out of the phone-hacking scandal, it has done the opposite, with weeks of revelations casting politicians as courtiers to the News Corp king.

The spectacle of a British leader answering hours of questions under oath at a time when he is grappling with a recession and the fallout from the eurozone crisis is a measure of how treacherous the scandal has become for Britain's elite.