Pubs, bars and restaurants may not open until May if the government take the advice of some scientists.
Dr Marc Baguelin, Imperial College London, who sits on the Scientific Pandemic Influenza Group on Modelling (SPI-M), a sub-group of SAGE, said the premature opening of the hospitality sector would lead to a "bump" in Covid-19 cases.
Speaking on BBC Radio 4's World at One programme, he said: "We looked at the partial reopening and the increase in the R number, it will generate an increase in the R number, the extent of which we don't know really.
"Something of this scale, if it was to happen earlier than May, would generate a bump in transmission, which is already really bad.
"So you have a lot of pressure on hospital, you will have another wave of some extent.
"At best you will keep on having very, very unsustainable level of pressure on the NHS."
Downing Street would not rule out the possibility of the lockdown continuing into the summer after Boris Johnson said it was "too early to say" when measures would be eased.
Ministers hope it will be the last national lockdown required during the pandemic.
Find your nearest vaccination centre by using your postcode below
But asked directly whether he could rule out the lockdown lasting into the summer, the Prime Minister's official spokesman said: "We will continue to keep all of the scientific evidence and data under review.
"It remains our position that we want to ease restrictions as soon as it is safe to do so, but in order for us to do that we need to see the transmission rates of the virus come down and we need to see the pressure on the NHS reduce."
