The new variant - known as P.1 - was detected by scientists circulating in Manaus, northern Brazil, in December.
It has several mutations in parts of the genetic code responsible for building spike proteins, which work like little grappling hooks to get into human cells.
Any changes to their design could make it easier for them to attach to human cells, while also making it harder for the immune system to get rid of the virus.
The Brazilian variant carries similar worrying mutations to the UK and South African versions, notably the N501 and E484 mutations that seem to allow it to transmit more easily.
But scientists are particularly concerned about the Brazilian variant because it has emerged in a population that was already approaching herd immunity, and should have been protected.
Research published last year suggested that 76 per cent of people in Manaus had contracted coronavirus by October, a level so high that it should have severely limited onward spread of the virus.
However the city saw an unexpected surge of new cases last month and has now declared a state of emergency, with hospitals reaching 100 per cent capacity.
A team of geneticists from South America, London and Edinburgh, found 41 per cent of cases in Manaus are now caused by the new variant.
Genetic sequencing has shown that at least one person caught the virus in the first wave has been infected with the new variant and scientists warned there had been a ‘rapid increase’ of cases in locations where previous infection rates were thought to be very high.
They have said it is now essential to investigate ‘whether there is an increased risk of re-infection in previously exposed individuals”.
Prof Sharon Peacock, the director of the Covid-19 Genomics UK Consortium (COG-UK), said the Brazilian variant was worrying because it shared mutation N501 with the UK variant which is believed to have increased transmissibility between 50 and 77 per cent. It also had other mutations which stopped antibodies from working.
“The fact the virus has the N501 means we know that it’s likely to be more transmissible, so that already brings alarm bells, and if it’s then carrying other mutations which could have other properties then we clearly need to take it seriously. But we have the tools to detect this and the public health capabilities to prevent spread.”