Figures from the Sanger Institute, which analyses positive swabs for different variants, show the India strains spread widely across England last month, accounting for around 2.4% of all infections in the week ending April 17.
That was a 12-fold rise from just 0.2% at the end of March.
Although numbers are still low, the figures suggest 10% of cases in the capital were caused by the B.1.617 India variants.
The proportion ranged as high as 46 % in Lambeth, South West London and 36 % in Harrow, North West London.
Scientists say it is not clear what is causing the rise.
The figures are based on tiny numbers of cases so clusters or super-spreading events have an amplified effect that may fade quickly.
However Professor Christina Pagel, a mathematician at University College London and member of the Independent SAGE panel of experts, said the India variant could be ‘outcompeting’ the Kent variant, which is dominant in the UK.
‘The numbers are still low but certainly in London right now, B.1.617 and its subtypes are the only variant that appears to be growing,’ Professor Pagel told MailOnline.
‘That could be because it is outcompeting other strains, including the dominant Kent strain, or it could be circumstantial in that there were some spreading events that happened that, just by chance, were the Indian strain.
‘However, I think the experience of India and now its neighbours do provide plenty of reason to be cautious and assume that B.1.617 is more transmissible.’
The UK has detected three different but genetically similar Covid variants which emerged in India, named B.1.617.1, .2 and .3.
All three are worrying scientists because they contain either one or two mutations in their spike protein that may help them evade the body’s immune responses and be more transmissible.
The variants have been linked to an explosion of cases in India that has seen dead bodies spill out onto the street and mass cremations take place in public car parks because hospitals have ran out of oxygen.
However there are too few cases in the UK to actually be able to tell anything about how the variants behave and not enough genetic testing in India.
The Sanger Institute data doesn’t include travellers’ tests and is intended to be a snapshot of community infection rates.
Scientists found 100 test samples with the Indian variants in the most recent week, up from 52 in the week to April 10.
During that time the proportion of national cases they accounted for rose from 1% to 2.4 %, while cases with the Kent variant went down slightly.
Professor Pagel said early modelling shows that it might well be more transmissible than the Kent strain.
She added: ‘We cannot be definitive. But that doesn’t mean we should be complacent either – as so often with Covid, waiting to be absolutely sure is waiting too long.’
The India variant has been found in dozens of local authorities across the country with hotspots in London and the Midlands.
Public Health England (PHE) has designated the Indian strains ‘variants under investigation’ because they are not well understood.
The Kent and South Africa variants are ‘variants of concern’ because they are known to spread faster and escape some types of immunity.
This means officials do surge testing to stamp out the South Africa variant when it’s found, but they don’t currently for India.
PHE’s Dr Susan Hopkins today said the agency is implementing tailored public health actions to detect cases of the India variant and mitigate the impact in local communities.
‘Enhanced contact tracing and testing is the most effective way of limiting spread,’ she said.
‘This precautionary approach ensures that our public health response remains agile and targeted. There is currently no evidence that the variant causes more severe disease or renders the vaccines currently deployed any less effective but more work is underway to understand that better.’
However, many scientists say the rapid rise in India cases means surge testing is more suitable than enhanced testing and tracing.
‘They absolutely need to be surge testing for it because it does seem to be spreading fast in the community,’ Dr Pagel said in an interview with the Guardian.
Meanwhile Paul Hunter, professor in medicine at the University of East Anglia, told the newspaper: ‘Looking at the most recent data, if surge testing has not yet already started then B.1.617.2 may already be spreading too widely for surge testing to be able to make a sufficient impact on reducing its further spread.’