Peter Kosminsky, the director of BBC2’s lavish adaptation of Hilary Mantel’s Wolf Hall books, has delivered an impassioned defence of the BBC.

Speaking on the same day that the Corporation set out its plans for moving BBC3 online – a decision which the BBC admits it was forced into because of financial constraints – Kosminsky used Wednesday night’s screening of the Tudor-set historical drama to urge the public to safeguard the organisation as it faces the squeezing of its budget.

“At its best the BBC is there to speak truth to power and that is something that every free society, every democracy badly needs,” he said. “And in my opinion that’s because of its unique funding because it is largely immune from the pressures of commercialism and because it exists at arms length, at least in theory, from government.

“To me at least it is a uniquely precious institution and one of the very few things of which we as a nation can be unequivocally proud and I beg you personally please don’t let us piss it away or, worse, let it go by default for some short term financial or political advantage. Because once it’s gone we will never get it back.”

Kosminsky (pictured, above, with Wolf Hall star Mark Rylance) referred obliquely to the BBC’s financial situation and did little to disguise his belief that the Corporation is under threat.

The BBC is engaged in identifying around £1.5bn in savings over the next two years as part of its obligations under the current licence fee settlement. It also faces an agonising two years' negotiating with the government before the renewal of the Corporation’s charter and the decision about the size of the new licence fee in 2016.

Kosminksy, whose dramas have included Warriors, The Promise and Britz, said: "There’s lots said about the BBC and about the licence fee. In fact it’s a subject on which almost anyone seems to have a strong opinion. I suppose the voices we don’t hear from often are the programme makers, so I’m going to say one or two words from the heart and I hope you’ll forgive me for that.

"For us the programme makers, it’s pretty straight forward really. The BBC is the captain of the ship. They are who we look to. They set the standards, and when they are absent from the bridge, as they occasionally are, we feel it particularly keenly.

“Before I started having the honour of working with actors I made documentaries for ITV for about ten years in some of the most troubled parts of the world.

“Because we were British, whenever we showed up people assumed we were from the BBC and to be honest we didn’t tend to disabuse them of that. It was a pretty honourable flag to sail under to be honest and the people we were dealing with tended to think that the BBC stood for honesty, stood for straight dealing and had high standards. Those were standards to which we tried to aspire.”

The screening was also attended by the drama’s stars – Damian Lewis who plays Henry VIII, Mark Rylance, who has taken on the lead role of his chief minister Thomas Cromwell, and Claire Foy who plays Anne Boleyn.

Rylance joked that the “codpieces were too small” when filming the drama and insisted that he did not read the history of the era and relied entirely on Mantel’s two books, Wolf Hall and Bring Up the Bodies, when working on the adaptation.

“I watched Carry on Henry, it was very good,” he laughed.

Homeland star Lewis said he had done more research into Henry VIII, including an analysis of his physicality.

“He remained slim and a formidable athlete long into his late thirties… and ballooned in the last ten years of his life in this rather Elvis-like fashion.

“I am not going to pretend that he didn’t have better legs than I do because he did. He was very proud of his calf, the shape of his calf, and he lorded it over Philip the Fair of France because he felt his calf was superior.”

Mantel was unable to attend the screening but she issued a statement praising the adaptation.

She said: "My expectations were high and have been exceeded: in the concision and coherence of the storytelling, in the originality of the interpretations, in the break from the romantic clichés of the genre: in the wit and style and heart.

"The spirit of the books has been extraordinarily well preserved. The storytelling is fast and fluid, the characters compelling, the tone fits that of the novels.

"Mark Rylance gives a mesmeric performance as Cromwell, its effect building through the series."

Read a first review of Wolf Hall, already tipped to be the best drama of 2015