In 1966, Britain's Labour prime minister, Harold Wilson, appointed George Brown as foreign secretary. Brown was a problem for Wilson; he was a senior figure in the party and popular with its rank and file, but he was also inept as a minister – he was sent to the Foreign Office to stop him damaging the economy as secretary of state for economic affairs. He was also a serious drunk, which was to prove his undoing.

Theresa May, the new British prime minister, had a Wilsonian difficulty when forming her administration. Boris Johnson is hugely popular with the Conservative party rank and file. Those who work with him (which at the moment include his fellow parliamentarians) tend to have less regard for his abilities. He is seen as unreliable, dishonest, lazy and with a capacity for saying things without weighing up the consequences.

This problem is exacerbated by the fact he has for years moonlighted in his original trade as a newspaper columnist, a calling in which punches are not pulled. Thus, he has described Hillary Clinton (who from next January he may be meeting regularly at international summits if she succeeds in becoming the head of state of Britain's chief ally) as resembling 'a sadistic nurse in a mental hospital', and has written a limerick in which he describes the president of Turkey having carnal knowledge of a goat, and in which 'Ankara' rhymes with 'wankerer'. Clausewitz he isn't.

It is widely assumed among Britain's political class, and the journalists who follow it, that May has executed a stroke of genius by putting Johnson in the Foreign Office. First, not even his most slavish supporter could claim that by giving him so great an office of state has she failed to reward adequately a man with such a huge political following. Second, by doing so she has shown solidarity with scores of MPs who felt Johnson was betrayed by his former lieutenant Michael Gove, who spectacularly withdrew support for Johnson during the recent leadership campaign and decided to stand himself (to be fair to Gove, he realised a few days into the campaign that his own credibility would be wrecked if he backed Johnson, given what he saw at close hand of Johnson's lack of capacity for organisation and reliability). Third, the Foreign Office is used to having products of Eton and Oxford at its helm (think Anthony Eden, Harold Macmillan and Lord Curzon), so Johnson was hardly a shot in the dark in that respect.

But, above all, Johnson has been given enough rope with which to hang himself.

Most of the important functions of the Foreign Office have been devolved elsewhere. Following Britain's decision to leave the EU, matters concerning that very delicate negotiation have been separated and apportioned to a new Department for Exiting the European Union, or Ministry of Brexit as it is more popularly known. A separate secretary of state, David Davis, is in charge of it, and answers to May, not Johnson. Sorting out the new trading relations that Britain must have with the rest of the world has been made the work of a new Department of International Trade, so that will be nothing to do with Johnson either. And, as has been the case since the Blair years, all the other most significant foreign relationships (with the US, Germany and China) and the management of tense policies (such as those concerning an aggressive, wayward Russia and the conflagration in the Middle East) will be run straight from 10 Downing Street.

'Boris's job,' another minister told me shortly after his appointment was announced, 'will be to fly round the world in a hot air balloon with a vast Union Jack painted on it waving a flag'. In other words, he will be a little like the celebrities sent to great shopping centres during November to turn on the Christmas lights; he is designed to cheer people up on behalf of the country, but not actually to engage in any serious act of foreign policy.

Despite his vast legacy of pronouncements recorded in his newspaper columns, we do not get what might be called a 'world view' from Johnson. The main purpose of foreigners was to provide him with copy, and with jokes such as the ones recorded above. Although he has a long record of being rude about the EU (and has that in common with many other UK columnists), nobody quite knew on which side of the argument he was going to come down in the recent referendum. When he announced himself as a Brexiteer, it was greeted with great cynicism, and with the assumption that he was on that side not out of principle but because on that particular day he thought Brexit was going to prevail.

When it did, he seemed shocked, realising that this might force him to run for the leadership of his party (which, briefly, he did) and then that he might end up in a serious job running a department of state. He had been mayor of London from 2008 until last May, but was legendary there for having had seven or eight deputy mayors to do the work for him.

He is broadly pro-American (he was born there and had an American passport until recently, when he surrendered it largely to avoid being taxed there) but, as we have seen, that does not prevent him being offensive about its possible next president. The problem with Johnson is that he has for years been an act, a species of light entertainment, whose womanising has become a national joke in the way that George Brown's drinking did half a century ago. Many in Britain will continue struggling to take him seriously. So, we must fear, will the rest of the world.

Photo: Flickr/Department of State