Covid-19 deaths could be twice as high over the winter as they were in the first wave of the pandemic, PM Boris Johnson is expected to warn MPs later.
In a Parliamentary statement he will say there is "no alternative" as he seeks to win support for the planned four-week lockdown across England.
But Mr Johnson will explain he was "right to try every possible option" before ordering people to stay at home.
Labour has said it will back the lockdown but criticised the delay.
Mr Johnson announced at a Downing Street news conference on Saturday that strict measures would be imposed across England from Thursday, closing pubs, restaurants, gyms, non-essential shops and places of worship.
Full details of the regulations are expected to be published before MPs vote on the measure on Wednesday.