It comes after his health secretary, Sajid Javid, admitted there could be 100,000 infections a day over the summer.
At Prime Minister’s Questions (PMQs) on Wednesday, Labour leader Sir Keir Starmer jumped on this and asked how many hospital admissions, deaths and instances of “long Covid” he is anticipating if cases hit this number.
Johnson refused to answer, claiming “there are a number of projections and they are available from SPI-M [Scientific Pandemic Influenza Group on Modelling] graphs”.
However, no such graphs have been made public under a scenario of 100,000 cases a day.
At Monday’s Downing Street press conference in which Johnson announced most Covid-19 mitigations will be dropped, chief scientific adviser Sir Patrick Vallance also refused to say how many deaths the 19 July relaxations are projected to cause.
Johnson continued: “It’s certainly true we are seeing a wave of cases because of the Delta variant, but scientists are also absolutely clear that we have severed the link between infection and serious disease and death.
“Currently there are only one-thirtieth of the deaths that we were seeing at the equivalent position of previous waves of this pandemic. That has been made possible thanks to the vaccine rollout, the fastest of any European country.”
Starmer hit back: “We know the link between infection rates and deaths has been weakened – but it hasn’t been broken.
“The PM must or certainly should know the answer to the question that I asked him. That he won’t answer it in the House hardly inspires confidence in his plan.”
Government data show infections are increasing exponentially. In the seven days up to Tuesday, 186,422 cases were recorded in the UK, up 49% on the previous week.
There were 28,773 daily cases reported – still more than 70,000 below Javid's summer projection.
While hospital admissions are nowhere near the levels seen in the second wave of the pandemic over the winter, they are still noticeably increasing.
Government figures show 2,092 Covid patients were admitted in the week up to 30 June, up 29% on the previous seven days.
There is a similar slow growth in Covid deaths: 142 were recorded in the seven days to Tuesday, up 20% week-on-week.
Starmer added: “Let’s be clear why the number of cases will surge so quickly – because he is taking all protections off in one go. That is reckless.”
The PM insisted "we will continue with a balanced and reasonable approach".