South Africa’s rising official inflation figures, which have sparked escalating industrial action and sent the cost of borrowing to a five-year high, are overestimated and based on “make-believe” calculations, a leading bank said on Wednesday.

Investec Asset Management said a delay in reforming the way in which inflation is assessed meant that the inflation rate – which hit 10.9 per cent in May – was two percentage points higher than it should have been.