A breakdown of cases indicates at least one case has been detected in Harrow and one in Brent.
It comes as surge testing for the South African variant continued in six London boroughs.
The Indian strain, VUI-21APR-01, has initially been classified as a variant under investigation rather than a variant of concern, pending a further risk assessment by PHE experts.
It was first detected in India, where there has been a 70 per cent spike in weekly Covid infections. India now leads the world in new cases, with more than 200,000 reported in the last 24 hours.
Dr Duncan Robertson, of Loughborough university, who has been analysing the spread of Covid-19, told the Standard: “We do not have full information on the 77 cases the variant under investigation first detected in India. We do know that at least one case has been detected in Brent and at least one case has been detected in Harrow.
“It is clear that variants under investigation and variants of concern are in England and that detected cases are rising.”
He warned that a spike in Covid variants – some of which may be more transmissible or harder for vaccines to defeat – could cause problems for the Government’s “roadmap” out of lockdown.
Experts have questioned why India has not been placed on the Government’s “red list” of countries that would ban travel to the UK. Boris Johnson is due to visit India at the end of April.
PHE declined to give precise locations for the 73 cases of the Indian variant detected in England but it is understood that they are spread widely and there are no reported clusters.
Sources indicated the variant may not need to be upgraded to a variant of concern, a process that would activate surge testing.
There have been 600 confirmed cases of the South African variant of concern in England, including 56 cases in the last week.
Surge testing has been launched across Wandsworth and Lambeth, and in parts of Southwark, Barnet, Hillingdon and Harrow. Thousands of testing kits are being sent to 20 schools in Wandsworth on Monday.