Health experts urged young Londoners to follow social distancing guidelines over the weekend of bank holidays, after the capital, saw an increase among people in their 20s in cases.
Professor Paul Plant, Deputy Regional Director for Public Health England for London, said that people in the 20 to 29 age group need to be careful and follow the guidelines.
He said this would help London “keep coronavirus at bay and avoid the possibility of a local lockdown”.
Public Health England issued a regional breakdown of positive test rates by age group in its weekly coronavirus monitoring report for the first time on Friday. It had previously only published age breakdowns for England as a whole.
The breakdown showed that the percentage rate per 100,000 young people aged 20 to 29 nearly trebled between the week of July 27 to August 2 and August 17 and August 23.
The positive case rate rose from around 10 per 100,000 people in the earlier period to about 27 per 100,000 people last week.
By comparison, last week, the 30 to 39 age group saw around 12 to 13 cases per 100,000 people, and those aged 40 to 49 around nine to 10 cases per 100,000 people.
Health protection authorities at Public Health England’s London office have also seen “an increase in positive tests in Londoners in their twenties in particular” in the past fortnight.
Professor Plant said: “Rates of COVID-19 in young people have been rising over the past two weeks in London, and the highest number of positive test results are now among people aged 20-29 across the city.
“This contrasts with overall rates for London boroughs which remain low related to other parts of the nation, but we are concerned about this rise in Londoners in their twenties.
“Lockdown has been a challenging time especially for young people, and we know many are now enjoying the benefits of being able to go on holiday, eat meals out, go shopping and see family and friends again, but we must do these fun things safely.
“Even if you are in a lower risk age group, please keep London safe for everyone by following government advice on social distancing, self-isolation, wearing face coverings and practising good hand hygiene so we can keep coronavirus at bay and avoid the possibility of local lockdown.”
It comes as the latest Government data revealed the range of the UK’s coronavirus R rate remains above one.
Data released on Friday by the Government Office for Science and the Scientific Advisory Group for Emergencies (Sage) shows the estimate for R across the UK is between 0.9 and 1.1.
If the R rate reaches one or above, the virus can spread exponentially and infect more and more people.
The growth rate of coronavirus transmission, which reflects how quickly the number of infections is changing day by day, has shifted slightly.
Scientists say there have been signs that these values may be increasing, with estimated ranges rising slightly from earlier publications.
For the whole of the UK, the latest growth rate is between minus 2 per cent and 1 per cent per day – a slight change from between minus 3 per cent and 1 per cent last week.