It goes against principles of diplomacy to say it, but Gordon Brown is right in his much-criticised boycott of this weekend's summit of European and African leaders simply because of the presence of Robert Mugabe.

Every normal calculation would conclude that he should show up. The Prime Minister says that he cares enormously about Africa; then surely he should be present at the most important European Union-Africa summit for years. In staying away, but failing to persuade anyone but the Czech Prime Minister to follow suit, he risks looking impotent and isolated himself rather than isolating the Zimbabwean President.

He has allowed Mugabe to gain the upper hand, to demonstrate the tacit support of his continent, and to give him all the advantages of being there. Nor, coming from Brown, after his petulant response to a month of crises at home, does the gesture carry the weight that it would from Tony Blair. Brown is thought by his EU counterparts to recoil from European gatherings in any case; given that perception, his response to this weekend's meeting risks seeming like a bad-tempered impulse rather than considered principle.

Despite all those good reasons for going, there are times when pure revulsion is enough justification. That reflex, in the presence of Mugabe, is the right one.

My colleague Martin Fletcher, in a series of long pieces from Zimbabwe, has caught in his reporting the reasons why no other attitude towards Mugabe should be possible. Development experts tend to talk of the country hurtling back down the chart of development, years of progress wiped out with each passing year. But that does not capture the stories of families of children orphaned by Aids, the untreated ordinary illnesses in hospitals without any medicines or anaesthetics whatsover, the annihilation of daily life as fuel and now food disappear altogether.

In 1980, when Zimbabwe became independent from Britain, life expectancy was 58; now it is the lowest in the world, 34 for women and 37 for men. It has the world's highest inflation rate — something like 15,000 per cent a year — another measure of the impossibility of normal life.

Brown was not entirely unsupported. Spain suggested that Mugabe may want to reconsider attending. Portugal, which issued the invitation, as holder of the EU presidency, ventured that it would be “preferable” if he didn't come.

But that is the limpest possible way of expressing an objection and, unsurprisingly, Mugabe dismissed it as provocation without weight. Other European leaders were even more worldly; Angela Merkel, the German Chancellor, while promising that Zimbabwe would not be “swept under the carpet”, argued that “this is such an important meeting that we should not let the presence of one country keep us from paying our respects to the rest of the continent”.

True, much of the continent is achieving astonishing change. But her “respects” should take account of the shameful solidarity of African leaders behind Mugabe. They prefer to characterise Western pressure as a neocolonial impulse, and defy it, than acknowledge that he has ruined Zimbabweans' lives — and ended many of them. He could attend the summit only because the 14 member states of the Southern Africa Development Community threatened to boycott it if he was not invited.

The EU, Africa's largest trading partner, is acutely conscious that its leverage on issues of governance and human rights is dwindling because of China's eagerness to invest, without such strings. But there is a point when such calculations should be set aside on principle. This is one.

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