The Prime Minister told a No 10 briefing the UK was at a “critical moment” and the rising number of cases and deaths shows “why our plan is so essential”, the BBC reported on Wednesday.
He said he would “not hesitate” to impose further restrictions if needed.
Chief scientific adviser Sir Patrick Vallance said: “We don’t have this under control at the moment.”
“There’s no cause for complacency here at all,” he added.
It comes as the latest UK coronavirus figures showed there have been a further 7,108 cases and another 71 deaths.
Last week, Johnson introduced restrictions including a 10pm closing time for pubs, bars and restaurants in England, with similar announcements in Scotland and Wales, and a 15-person limit on weddings.
Since then, further local lockdowns have come into force, including in north-east England, where households are banned from mixing indoors.
At the press conference at Downing Street, Johnson also said the nation could face the winter “with confidence” because it was now better prepared than in March.
The preparations include being on track for 500,000 tests a day by the end of October, 2,000 beds in seven Nightingale hospitals and a four-month supply of protective equipment (PPE) such as masks, gowns and visors.
He said they had trebled the number of ventilators in the NHS to 31,500 in the last six months.
There were 312 Covid-19 patients in mechanical ventilator beds as of Tuesday, the government said, and 2,252 in hospital, as reported on Monday.