Speaking hours after government scientists lifted the Covid alert level to 4 on a 5-point scale, Johnson said the booster programme must go faster because scientists did not yet know if Omicron was less severe than other variants.
"A tidal wave of Omicron is coming," Johnson said in televised statement on Sunday evening (local time). "And I'm afraid it is now clear that two doses of vaccine are simply not enough to give the level of protection we all need."
He added that scientists knew Omicron was much more transmissible and that the National Health Service would struggle to cope with hospitalisations if the variant were to tear through an un-boosted population.
"Everyone eligible aged 18 and over in England will have the chance to get their booster before the New Year," he said.
Health Secretary Sajid Javid reiterated the warning on Monday, saying Omicron was spreading at a "phenomenal rate".
Though Javid said there had been no deaths yet confirmed in England, he said Omicron was probably behind about 40% of infections in London and that unless the government acted then the health service could be overwhelmed.
"What we now know about Omicron is that ... it's spreading at a phenomenal rate, something that we've never seen before, it's doubling every two to three days in infections," Javid told Sky News.
"That means we're facing a tidal wave of infection, we're once again in a race between the vaccine and the virus."
Last week Johnson introduced "Plan B" Covid-19 measures for England, ordering people to work from home, wear masks in public places and use vaccine passes to slow the spread of Omicron.
With many of Johnson's own Conservative Party lawmakers set to vote against these measures, Johnson has said there are no plans for further restrictions as cases soar.
The seven-day average of Covid cases by date reported has risen in recent days above 50,000 - the highest since the January peak during the last wave of cases.
With 146,439 fatalities recorded within 28 days of a positive Covid-19 test as of Sunday, Britain has Europe's highest death toll from the virus.
While that damaged the government's reputation in the depths of the pandemic, officials have been praised for rolling out one of the fastest vaccine programmes, which helped to contain daily death figures.
In recent weeks however questions have once again emerged over Johnson's future as prime minister after a series of scandals, the most damaging being reports that a party was held at Downing Street during a 2020 Christmas lockdown when such festivities were banned.
A Survation poll for the Daily Mirror newspaper published on Sunday found that 77% of Britons said they were less likely to follow Covid rules if government officials had broken them.
