The prime minister also highlighted the potential danger posed by a new Brazilian variant, as he gave evidence to a committee of MPs.
He highlighted how the 32,000 coronavirus patients currently in hospital was “about 70 per cent more than there were in April last year”, at the start of the pandemic.
And he warned: “If you asked me, when do we think that the ICU [intensive care unit] capacity is likely to be overtopped I can’t give you a prediction for that – but all I can say is that the risk is very substantial.”
Describing the “colossal” strain” on NHS staff, the prime minister added: “We have to keep the pressure off the NHS and the only way to do that is to follow the current lockdown.”
Speaking to the Commons Liaison Committee, Mr Johnson also suggested workers making vaccines should be fast-tracked to receive jabs, after fears of high absence rates.
And he agreed it was “absolutely essential” that local health directors know who has been vaccinated in their areas, saying it will happen “by the end of this week”.
As he was speaking, the University College London published a study warning about increasingly-full intensive care units – and the increased likelihood of admitted patients dying.
When units reached 85 per cent capacity, 38.4 per cent of patients died – a 19 per cent increase on normal and the equivalent of being up to 11 years older.
“To put into context as to why these results are so important , at the end of 2020, 37 of 123 recorded NHS trusts were experiencing over 85 per cent ICU bed occupancy,” said the study’s author, Dr Bilal Mateen.
“Eleven of these were completely full and the situation has only got more extreme.”