The bitter truth is that no major figure in British public life was prepared to take on and expose the Murdoch newspaper empire. Rival proprietors were silent. Senior public figures did not dare to speak out for fear of exposure and attack in the Murdoch newspapers. This is why, for more than a generation, Rupert Murdoch’s empire has been a spider at the heart of an intricate web that has poisoned British public life.

Murdoch used his power to immense effect, undermining and attacking our greatest public institutions – above all the monarchy and the judiciary. His employees believed they were above the law and could act with impunity. And this was indeed the case, thanks to the connivance of police, politicians and the press.

But now – brought down almost single-handedly by the brilliant Guardian campaigning journalist Nick Davies – Rupert Murdoch’s web is unravelling. The media tycoon is fighting for his survival, and as his empire is rocked the reputation of his political client base is collapsing, too.

Most devastating is the damage done to David Cameron. I can reveal exactly how Cameron was, against his better judgment, drawn into Murdoch’s inner circle – thus building a range of associations that have brought disgrace to 10 Downing Street, the British government and Mr Cameron himself.

To begin with, Cameron was wary of Murdoch. His first meetings with the tycoon went badly. After one meeting, a senior News International figure complained to me: “We told David exactly what to say and how to say it in order to please Rupert. But Cameron wouldn’t play ball. I can’t understand it.”

Cameron had made the deliberate decision to gain power without Murdoch’s assistance. Urged on by his senior aide – and probably his closest political friend, Steve Hilton – the future prime minister kept his distance.

But this strategy led to disaster in the polls. David Cameron was mocked and ridiculed in the Labour supporting Murdoch press, and by the summer of 2007 matters reached a crisis. There was talk that Gordon Brown, newly elected as Labour leader and Prime Minister, would call a snap election that autumn which he was widely expected to win handsomely.

It was at this point that George Osborne, then shadow chancellor and also Cameron’s closest strategic advisor, entered the fray. The immensely ambitious Osborne – who was already cultivating his own links with News International – made the case that Cameron should hire Andy Coulson.

Coulson was a brilliant News of the World executive, hand picked by Murdoch himself to go to the very top of the News International organisation. But his career had met with a setback a few months previously when he had been forced to resign as editor after the royal reporter Clive Goodman was sentenced to jail for hacking into the mobile phones of members of the royal household.

Cameron accepted Osborne’s view that there was no need to worry about this blot on Coulson’s record. This turned out to be a fatal miscalculation. Disastrously, Cameron imported Coulson into his inner team of advisors. In the short term, Coulson proved to be an excellent decision. He gave sound strategic advice, which helped Cameron see off the threat from Brown and enjoy a remarkable recovery in the opinion polls. But Coulson also performed one other function. He helped draw Cameron deep into the inner circle that surrounds Rupert Murdoch. In particular Cameron allowed himself to become a member of what is now known as the Chipping Norton set, a group of louche and affluent Londoners who centred around Rebekah Brooks’s Oxfordshire home, barely a mile from Cameron’s constituency residence.

Soon News International, through Coulson, had a key say in Conservative Party decision-making and even personnel appointments. It was News International, once again acting through Coulson, which effectively ordered Cameron to sack Dominic Grieve as his shadow home secretary in the autumn of 2008. Grieve was duly reshuffled in January 2009, after less than a year in the job. The irony of that decision is bitter today, for the decision given by News International for wanting Grieve out was that he was too soft on crime. Finally Cameron’s friendship with News International delivered the ultimate prize – the support of the Sun in the 2010 general election.

By now Cameron was entranced with Coulson and so addicted to his advice that he made the decision that may define his premiership, and that he will regret till the day he dies. He took the former News of the World editor into 10 Downing Street, giving him the very senior role of Director of Communications. Many warned Cameron against this decision at the time and it has turned out to be a catastrophe for him.

And even now – not even at yesterday’s press conference – Cameron is unable to grasp the truth of the situation. He is still acting within the same dreadful moral parameters that have infected public life since the emergence of Murdoch more than a generation ago. This is why Cameron is still unable to extract himself from the sewer in which – as of last night – he is still swimming. To do so he must now take head-on the media giant News International and its disgraced senior executives who helped propel him into Downing Street.

At the press conference, the Prime Minister failed to take the vital step needed if he is ever to break free from this sordid business. He must refer to Ofcom, not just the proposed Murdoch consolidation of BskyB, but all the Murdoch business in Britain to see if Rupert Murdoch himself, in the light of the disgraceful conduct of his businesses, passes the “Fit and proper person” test for ownership of a British public company.

To say, as Cameron did, he would have accepted Rebekah Brooks’s resignation is simply not enough. Cameron urgently needs to follow the example of Labour leader Ed Miliband and call for her to step down as Chief Executive of News International. Burning questions also concern the conduct of Murdoch’s son, James, who signed off what looked very much like hush-money payments to prevent embarrassing material entering the public domain. There is a reason for Cameron’s inaction. Fundamentally there are two factions inside his government. There is the pro-News International faction led by Osborne. Its other members include Michael Gove, the Education Secretary, Jeremy Hunt, Culture Secretary, and Hunt’s deputy Ed Vaizey. Andrew Cooper, recently awarded a senior job as director of strategy in Downing Street with the support of Osborne, is another figure in this camp.

It is important to bear in mind that when Osborne threw his 40th birthday party at Dorneywood several weeks ago, the place was apparently knee deep in News International figures. Another example is even more recent. Only last weekend Matthew Freud, the PR tycoon who is married to Rupert Murdoch’s daughter Elisabeth, threw a party in his Cotswold home. Michael Gove and Ed Vaizey – both semi-detached members of the Chipping Norton set – were both in attendance. Showing characteristic lack of judgment, Vaizey was reportedly seen at this party deep in conversation with James Murdoch.

There are members of the Cameron circle who have always warned against the consequences of the Murdoch alliance. Chief among them is Steve Hilton, who, according to friends, has been driven to despair by the Murdoch influence inside Downing Street. Another sceptic has been Nick Clegg, the Deputy Prime Minister, who sadly has been too weak to speak up.

Fundamentally David Cameron, like many of us perhaps, has something of a split personality. I have known the Prime Minister for 20 years and feel certain that he is genuinely filled with sound and decent values imbued in him by his parents, in particular his mother, a former Justice of the Peace.

But there is also another side to the Prime Minister: a sharp-suited media operator who will do what it takes to get to the top. Sadly, as last year’s election loomed, it was the ambitious and amoral side of David Cameron that took command. He is now living with the consequences.