I was disappointed to find that the French president’s remark this week, comparing a difficult diplomatic conversation with Donald Trump to a sausage (because it’s best not to know what’s inside), goes back to Bismarck. I thought President Macron had made an original joke about diplomacy that could survive translation into more or less any language – something not many world leaders have managed to do.

I do not consider it possible that there was something really disgusting in the Macron-Trump sausage, something that would cause legitimate revulsion if it was revealed. That is possible in diplomacy, of course – but, thank God, it is rare. An example would be the Anglo-French-Israeli collusion to go to war with Egypt in 1956; the British and French governments destroyed their records, but fortunately for history the Israelis did not. Another example might be the Blair-Bush exchanges about war in Iraq.

Serious exchanges between statesmen and between diplomats often have to be kept private. That is not unique – the same is true of conversations between journalists, businessmen, and indeed people in their ordinary lives. The difference with conversations between diplomats is that they have to be kept private more often than not, because the subjects they deal with may be important and controversial, and typically affect third parties. When I was UK ambassador in Luxembourg I was asked by the head of the monetary agency to pass a warning to the Bank of England that the Bank of Credit and Commerce International, officially regulated in Luxembourg but in fact operating in London, was heading for the rocks. He was right; it was closed down not long afterwards.

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Often the need for privacy is only temporary. In the 1980s “confidential” papers I had written in the Foreign Office were betrayed by a spy to the Egyptian government. They concerned negotiations we were conducting with Egypt about a proposed multinational force in Sinai. I was asked for a “damage assessment”. I advised that the leak was in fact beneficial to our government because it would have proved to the Egyptians that we were negotiating in good faith.

In my experience of government in Whitehall and elsewhere, leaks don’t tend to come from officials, senior or otherwise. Unless there is espionage, which is rare, they usually come from ministers. As the Turks say, the fish stinks from the head – but that is not the whole story. As a civil servant, I have no authority to go public; as an ambassador I have very limited authority; as a minister or prime minister or president, I do what I like and take the consequences. Most heads of government or state still guard their tongues. The Trump phenomenon is an unusual and unsettling one because Trump makes a virtue of uncertainty. What he says tomorrow may be quite different.

Play Video 0:47 Macron: phone calls with Trump are like sausages – video

The position of a diplomat is special, because most of the important things they say on the record are not simply their own personal opinion but the position of the government they represent. The US ambassador to the UN, Nikki Haley, introduced herself to the UN by saying that unlike other ambassadors she likes to say what she thinks, not what she is told to say. My reaction as a professional is that there is therefore no need to bother with her.

There is the difficult question of trust. Curiously enough, diplomats from different countries normally trust each other in a certain sense. They know, of course, that each is loyal to a different government. But they also know that their business is to provide a channel for confidential communication between the two governments, and that inappropriate leaking has a cost, which they are probably not authorised to pay.

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Colourful language is occasionally needed, but usually not. When I was in Luxembourg I was instructed, along with my colleagues in other European capitals, to explain to the government there that Britain could not accept a compromise proposal about Gibraltar that would involve flying a Spanish flag. Of course my Luxembourg counterpart argued the other case – a flag is just a flag. Improvising beyond my instructions I asked him whether he could accept a solution to a local problem, for example on the motorway running through Luxembourg, that would involve flying a German flag over Luxembourg (Luxembourg still remembers that it was not only occupied by Nazi Germany but incorporated into the Third Reich). “Of course not,” he snapped back. “That would be quite different.” As the lawyers say, I rested my case.

Another example: I was UK ambassador to Libya when Britain severed diplomatic relations in 1984 after the murder of the police constable Yvonne Fletcher in a shooting outside the Libyan embassy in London. For some reason I now forget, I was instructed to hold up agreement on the question of appointing “protecting powers” to look after each side’s interests. The Libyans said their protecting power would be Saudi Arabia. I asked if Saudi Arabia had actually agreed to that – the answer was no, but it was a friendly Arab state, and therefore this was a foregone conclusion. “No,” I said. “As you know, the Saudi flag has the name of Allah on it. They might not wish it to be used to cover up the murder weapon.”

“Mr Miles, there was no murder weapon …” came the reply. But they knew that there was.

• Oliver Miles is a former diplomat