At the start of February only 150 cases of the new variant had been discovered in the UK.
The Nigerian strain, labelled B1525, had infected fewer than 100 people at the end of February.
However, the number of infections surged to around 250 by the end of March and 300 by this week.
A senior Sage source told iNews: “If it can infect and transmit in vaccinated people, B1351 will increase in that population.
“It is slowly going up, but that might be because we are looking for it so hard.”
In its latest report on the advance of the coronavirus pandemic, Sage warned the South African variant remained of significant concern in the UK.
It said: “The widespread transmission of a variant that circumvents vaccine-induced and naturally acquired immunity is a realistic possibility.
“Some parts of continental Europe have relatively high prevalence of B1351, and this variant in particular is of great concern, given the known reduced protection against mild to moderate disease from certain vaccines for this strain.