The UK has recorded 223 COVID-related deaths within 28 days of a positive test in the biggest daily jump since early March.
A further 43,738 coronavirus cases were also recorded in the latest 24-hour reporting period.
It is the highest daily fatalities figure since 9 March, when 231 people died within 28 days of a positive test.
And last Tuesday, there were 181 deaths and 38,520 cases recorded.
The figures come after the government said it was also keeping a "close eye" on a new mutation of the Delta variant, which has been called AY.4.2 by scientists.
The UK Health Security Agency warned this variant is now making up 6% of new cases and could be up to 10% more transmissible.
Adam Finn, a professor of paediatrics at the University of Bristol and a member of the Joint Committee on Vaccination and Immunisation (JCVI), says the "very rapid rise" in the number of cases in the UK is a "reflection of how people are behaving".
He told Sky News "nobody" appears to be wearing masks inside anymore, adding he doubts people are doing lateral flow tests.
Professor Finn said: "There's a general sense that life's gone back to normal and as a result the infection is being passed around."
He added that unless there is a "clear message put out that that we've got more infection going on now than at any point in the pandemic", despite fewer hospitalisations, people are "not really going to take precaution".
Asked if he expects another lockdown, he says: "I sincerely hope not.
"I do think we need to make a bit more of an effort to communicate to people that unless they all make an effort to reduce transmission of the virus it's going to go on circulating and being a problem.
"We can all contribute to doing that without having a lockdown or massively disrupting our lives."
He also defended the vaccine rollout, saying it's going "forward as fast as can be arranged".
Prime Minister Boris Johnson's official spokesman said the government is "not complacent" about the rising cases.
He added the level of hospital admissions and deaths were still "an order of magnitude lower" compared to the earlier days of the pandemic.
The spokesman also said this afternoon there were currently "no plans" to use the contingency measures set out in the autumn/winter strategy.
He went on to stress that "the most important message for the public to understand is the vital importance of the booster programme".
Meanwhile, Health Secretary Sajid Javid said that the government is "ramping up" its vaccination programme in a bid to encourage eligible children to get jabbed.
He added children aged between 12 and 15 years wouldbe able to use the national booking service to secure their COVID-19 vaccines "to make the most of half-term next week".
Mr Javid said: "To make the most of half-term next week, we will now be opening up the national booking service to all 12 to 15-year-olds to have their COVID vaccinations in existing national vaccination centres, which will offer families more flexibility.
"I think it is important that anyone who is invited as eligible for a vaccine, including young people, that they do come forward and take up that offer."