In 1985 Clive Ponting, a senior Ministry of Defence civil servant, was prosecuted under the Official Secrets Act for passing a note to a Labour MP revealing that the Thatcher government was misleading parliament over the sinking of the Argentine cruiser, the General Belgrano, during the Falklands war.

MoD officials had agonised over whether they should ask the police to conduct a leak inquiry. They decided to do so after discussing the matter with the defence secretary, Michael Heseltine. They noted that just two years earlier a junior official at the ministry, Sarah Tisdall, had been jailed for passing documents to the Guardian about US cruise missiles being based in Britain and it would look bad if a more senior official was not prosecuted.

Ponting was unanimously acquitted by a jury at the Old Bailey. It was an early sign that jurors were getting less deferential, in this case even ignoring the judge’s direction to convict.

That has not stopped governments from demanding prosecutions when they thought, as they did in the case of the young MI5 renegade David Shayler, that jurors would have little sympathy with a leaker’s motives. In one notorious case, the security and intelligence establishment belatedly realised they had made a big mistake. Early in 2003 Katharine Gun, a young GCHQ translator, leaked a document, subsequently passed to the Observer, showing how the US had asked GCHQ to bug the phones of diplomats from the so-called UN “swing states” – countries believed to be open to persuasion to back an invasion of Iraq.

Facebook Twitter Pinterest Clive Ponting was acquitted by a jury at the Old Bailey in 1985 after a leak about the sinking of the Belgrano. Photograph: Peter Brooker/Rex

She was charged under the 1989 Official Secrets Act which had replaced the discredited “catch-all” statute. The case against her was suddenly dropped when the prosecution realised that evidence would emerge at Gun’s trial that even British government lawyers believed the invasion was unlawful.

But all recent official secrets cases show that Tom Watson, Labour’s deputy leader, and others were right last week to suggest that had the finger of suspicion over the National Security Council leak about ministers’ differing views on Huawei, the Chinese telecoms company, pointed to an official, rather than the defence secretary, Gavin Williamson, the police would quickly have been called in.

A few ministers have resigned over a leak – Leon Brittan, the trade and business secretary, was the last one to do so during the 1986 Westland affair. No minister has been prosecuted under the Official Secrets Act and it is easy to understand why.

Such prosecutions require the approval of the attorney general after consulting other members of the government. Prosecutions under the act are a matter of political discretion.

Facebook Twitter Pinterest Katharine Gun, a translator at GCHQ, was charged with breaching the Official Secrets Act about the Iraq war in 2003. Photograph: Martin Godwin/The Guardian

May told Williamson there was “compelling evidence” he was responsible for the Huawei leak. After sacking Williamson, she insisted the matter was closed. The last thing she wanted was prolonged argument and embarrassing evidence emerging before or during a trial allowing Williamson to talk of a “witch-hunt” and a “kangaroo court”, and that he would be “absolutely exonerated”.

That would also examine the way the leak inquiry was handled by Mark Sedwill, the cabinet secretary and national security adviser, a man who has crossed swords with Williamson over defence spending, with Sedwill arguing for more money for spooks rather than soldiers.

Sedwill had no more interest in a police investigation than the prime minister. Moreover, attempts would be made to question Daily Telegraph journalists, raising the whole question of the freedom of the press.

“You leak, but I brief,” the former Labour prime minister, Jim Callaghan, told his civil servants. One notable example was how the Ministry of Defence press office was authorised by ministers, in an elaborate stratagem, to name David Kelly, the government scientist, as the source of the BBC journalist Andrew Gilligan’s report about the discredited Iraqi weapons dossier. There are more subtle ways journalists with the help of friendly ministers can put two and two together. This may help to explain Williamson’s denials.

It may have been the first leak directly attributed to a particular meeting of the NSC. But it is not the first time information on matters discussed at the NSC has been leaked. Concerns expressed about the legality of targeting Muammar Gaddafi during the air strikes against Libya in 2011 were reported. Rows between armed forces chiefs over where cuts should fall have regularly leaked. Special forces operations are in theory protected by a wall of official secrecy. In practice they are leaked, not least by members of the special forces themselves.

Facebook Twitter Pinterest Sarah Tisdall leaves Bow Street magistrates’ court in 1983 on charges of leaking nuclear secrets to the Guardian. Photograph: Bill Cross/Rex

Journalists have reported arguments within MI6 and between MI5 and MI6 before the NSC was set up but on sensitive matters that would now come before the NSC. The way Tony Blair and his aides used and abused intelligence before the invasion of Iraq was leaked wholesale by whistleblowers in what became known as the Downing Street Memos.

No “secret intelligence” was leaked from the NSC discussion about Huawei. But for an ailing prime minister battered by a cabinet deeply divided by Brexit it was a leak too far. It was not about national security, but about trust and about an individual who had already tested his cabinet colleagues and senior officials by aggressive ill-judged rhetoric. Leaks will always be used as weapons and they are often the result of excessive secrecy. The Official Secrets Act will remain on the statute book, but as a potential threat to officials, especially junior ones, not to ministers.

The State of Secrecy by Richard Norton-Taylor, will be published by IB Tauris early next year