The Leveson Inquiry's exposure of endemic bribery of police and public officials by some News International journalists was shocking enough. But yesterday's revelations of what appears to be a "back channel" between the media group and the cabinet minister charged with ruling on its highly controversial plan to take over BSkyB cast even that into the shade. In the circumstances, the position of the Culture Secretary, Jeremy Hunt, is untenable. He must resign.

The picture painted by the 163 pages of emails handed over by News International as James Murdoch took to the witness stand yesterday is a highly disturbing one. The documents suggest an almost daily contact between Mr Hunt's office and News Corp's head of public affairs, Fréd Michel, providing a running commentary on the progress of the £8bn BSkyB deal.

According to Mr Michel, Mr Hunt's office told him that the Government would be "supportive throughout the process", cooked up a plan that would mean "game over for the opposition", and even leaked information ahead of a statement by Mr Hunt to Parliament, with an (apparently joking) aside that such practices were "absolutely illegal".

Given that Mr Hunt had the quasi-judicial task of ruling on a corporate takeover that would materially affect not only the company itself, but Britain's entire media landscape – with all that that implies – it is difficult to overstate the seriousness of yesterday's revelations. It is one thing for a lobbyist to put their employer's case, and for government ministers and officials to hear it. But what has been exposed here is something else entirely; indeed it looks little short of collusion. Even worse, Mr Hunt was only given the responsibility after Vince Cable was caught out by a newspaper sting boasting that he had "declared war on Rupert Murdoch" and removed from the role because of his perceived bias.

The Culture Secretary must now show he was not biased the other way.

Mr Michel stated explicitly that the "JH" in the emails was shorthand for the cabinet minister's office, rather than the man himself. And Mr Hunt will be at pains to prove not only that he never had any dealings with Mr Michel, but that he took legal counsel throughout, and that either his political advisers went too far, or the lobbyist was wildly exaggerating to his bosses, or both. Indeed, Mr Hunt's statement last night claimed discrepancies in Mr Michel's evidence. The Culture Secretary also said he was confident the public would see he conducted the process with "scrupulous fairness".

In fact, the minutiae do not matter. Even if Mr Hunt was not in direct contact with News International himself, he must take responsibility for the wholly inappropriate activities of his staff. After all, this was no single incident, this was pages and pages of emails, an entire relationship of such staggering impropriety as to leave Mr Hunt yesterday charged with acting as a "cheerleader" for News Corp within the Government. Fair or not, the damage is done and he cannot credibly continue in the job.

The Prime Minister was last night sticking to the line that he has full confidence in his Culture Secretary. That this is the same David Cameron who, before the election, warned that corporate lobbying was "the next big scandal"only adds to the sense of – in Mr Cameron's own words – "a cosy club at the top making decisions in their own interest". And Mr Murdoch's admission yesterday that he did briefly mention the takeover to the Prime Minister, at a dinner held by Rebekah Brooks, hardly helps.

For all yesterday's dirt, there may yet be more to come. When Rupert Murdoch follows his son to the witness stand at Leveson this morning, he may take the opportunity to turn the tables on the political class that has tried so hard to hang him out to dry. Either way, if Mr Hunt will not resign, Mr Cameron has no option but to sack him.