Guests streaming out of No 10 on a sunny evening in May 2007, after a farewell party hosted by the departing Tony Blair, may have noticed a black Bentley outside No 11. The windows were blacked out. "Murdoch," said my partner Jonathan Powell, Blair's former chief of staff. Of course! Hardly had Tony Blair packed his bags than Rupert had dropped round to see the new man, Gordon Brown.

My first encounter with Murdoch's brand of power came 21 years earlier in the Sunday Times newsroom, where I was covering the print unions as they geared up to fight Murdoch over the introduction of new technology. For a year he had been constructing his Wapping plant, where journalists were to be moved after the printers had been sacked. But so secret was the Wapping plan that I had missed the story – along with everyone else.

The first I knew of the Wapping move was when I returned to the newsroom one Saturday evening in January 1986, only to find the place deserted and every journalist in the paper packed in a smoky room upstairs. Here, the editor, Andrew Neil, sat on a desk, next to an Imperial typewriter, telling the staff that they all had to go to Wapping by Tuesday or be sacked. It was a dramatic scene. Some of the finest journalists in the country – Ian Jack, Don Berry, Claire Tomalin, David Blundy – sat open-mouthed with the shock. Over the weekend, endless NUJ meetings were held, as we had to face up to the decision: to go or not to go to Wapping.

There were nervous breakdowns and impassioned speeches on all sides. Peter Murtagh, an Insight team member, said he would never go: "The editor does not tell me I am part of the greatest team of journalists in the world and then hold a gun to my head." What hope was there for journalism in Murdoch's Wapping, asked others?

What I remember most clearly about the meetings about these heated debates, however, was not so much the debates of rights and wrongs about Murdoch's objectives: many – even among this liberal group – knew modernisation had to come. What shocked people was the audacious brutality of Murdoch's methods. Nobody had predicted Wapping because nobody had ever imagined such a radical plan. The extraordinary secrecy and lies that had masked the construction of his wired-off camp had now been stripped away to show a readiness to cleanse an entire workforce. If they didn't go along with this, journalists were to be thrown on the heap, too. Today, at the News of the World, another band of journalists are in shock at Murdoch's methods.

In 1986 some saw immediately that collaboration with Murdoch had to stop then and there. Claire Tomalin, then literary editor, saw it perhaps clearest of all. The ultimatum, she said, was "the most appalling thing she had ever heard" and she resigned. Others, including me, didn't yet believe what he was capable of and decided to give Wapping a try. I had only been on the paper for 18 months and it was the job I had dreamt of all my life.

The following Tuesday I went to Wapping, but no sooner was I behind the wire than I knew it was a terrible mistake. The site was a soulless place, where the word "consultant" was written on our ID cards instead of journalist, and instead of working for the Sunday Times we now worked for "Tower Hamlets Plant". Ashamed to admit that I was calling from "behind the wire", I found I could barely ring out to research stories. There was no library here, which showed just how much our new bosses cared for the concept of research. The desperate look of pleasure on the faces of those already there each time a new collaborator arrived to join them said it all.

As the weeks passed and the picket lines heaved outside, a distasteful culture began to take hold behind the wire and it became clear that a more cut-throat kind of journalism was expected. After driving through the picket lines in my Honda Civic for three months (at least I never took the grilled buses), I finally escaped to the Independent, founded in October the same year.

In recent years I have seen something of how Murdoch's power then evolved. As wife of one of Tony Blair's aides, I was aware of the constant concern with how New Labour stories were playing in the Murdoch press. Murdoch's own visits to No 10 were far more frequent than visits from any other newspaper proprietor, as were his phone calls to Blair, usually unmonitored. The Murdoch papers' loathing for Europe was without doubt behind Blair's cooling towards Brussels and the euro. By 2003, such was Murdoch's access to Blair, that he was even able to put a call through to the prime minister on the eve of the Iraq war and advise Blair to join George Bush's invasion.

And all this time, the courting of News International executives gathered pace. During the cash-for-peerages affair, the Murdoch papers attacked Blair and his circle day after day, sometimes writing entirely fanciful tales. Yet the likes of Rebekah Brooks and James Murdoch were still being invited to parties thrown by senior Blair aides.

We all had a taste of some of the worst of News International's journalistic methodology. When Jonathan's notes on Downing Street notelets, taken from our dustbins, appeared in the Sunday Times one day it seemed like a harmless episode, but the amoral audacity of it was a Murdoch trademark and a sign of the phone-hacking scandal to come.

Andreas Whittam Smith, founder of the Independent, suggested last week that Murdoch had begun to pose a threat to British society something like the mafia in Italy. On the same day, a News International editor, trying to explain why Brooks had not been sacked, said that she was treated by Murdoch as "family".

There was certainly something of The Godfather about that black limousine rolling into Downing Street in 2007. Is there a hope now that in future when the family Bentley rolls into Downing Street, it will be firmly turned away?

Sarah Helm's play, Loyalty, opens at the Hampstead Theatre, London, on Thursday.

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