A London borough has urged residents to avoid mixing with other households “unless absolutely necessary”.
It comes amid fears coronavirus is spreading more rapidly in the capital than official figures show.
In an open letter, the mayor of Tower Hamlets in east London John Biggs wrote: “Despite a fall over the summer, we are seeing cases of Covid-19 rise and we need to accept that the situation is once again worsening. Tower Hamlets now has one of the highest levels of Covid-19 in London.
“As a second rise in infections hits us, we must take all steps necessary to limit the spread of the virus and protect those most at risk.”
He added: “With this in mind, now is the time we must take further action. I am clear that the current national rules are a minimum and my advice to you all is to do everything in your power to protect each other. Our individual actions have consequences for us all.
“The next few months will be very challenging. Without a vaccine or more effective treatment, our primary weapon against the virus is responsible behaviour. Measures to curb the spread of the virus will only work if people follow them.
“In addition to the national measures, we're asking residents to help reduce the spread of Covid-19 locally.
“Many cases of Covid-19 in Tower Hamlets are as a result of people visiting other households, so we are asking residents to avoid this unless absolutely necessary.
“This really is a matter of life and death and we need to act urgently.”
The move by Tower Hamlets comes as ministers have face renewed calls in recent days for the imposition of local lockdown restrictions.
Earlier this week, Jas Athwal – the leader of Redbridge Council, the London borough with the highest infection rate – said cuts to testing capacity in the city meant the true picture was being “distorted”.
He called for an immediate ban on different households mixing in the most overcrowded areas to prevent the situation getting worse.
“Quite clearly I think in overcrowded parts of London we have got to be looking at bringing that in so that different households can’t mix,” he told the BBC Radio 4 Today programme.
“Mixing face-to-face should be stopped immediately because there are problems and we are seeing there the pandemic take hold. It is only going to get worse with the flu season coming into play as well.”
The warnings came after London mayor Sadiq Khan issued a call last weekend for immediate action to stem the spread of the virus in the city, saying the capital was at a “tipping point”.