In a pre-recorded message, televised at 8pm tonight (Sunday, December 12), Mr Johnson said: "We are now facing an emergency in our battle against the new variant."
The Prime Minister said the Omicron variant is 'doubling every two to three days' and two doses of the vaccine are 'simply not enough to give the level or protection we all need'.
The PM urged the country to make an 'extraordinary effort' to meet the new booster target.
He said each day, the country will need to beat NHS’s best vaccination day yet.
In a bid to offer the booster to all adults by the new year, the NHS app will allow bookings to be made for all adults from Wednesday but walk-ins will be accepted as early as Monday December 13.
"Today we are launching the Omicron Emergency Boost," he said.
"A national mission unlike anything we have done before in the vaccination programme — to get boosted now."
Mr Johnson warned that people shouldn't be complacent and not expect themselves or their families to get seriously ill with the new variant.
To meet the new targets of the booster rollout programme, the PM confirmed the government will be; deploying military planning teams across every region; setting up additional vaccine sites and mobile units; extending opening hours so clinics are open seven days a week and training thousands more volunteer vaccinators.
It comes after the UK Covid Alert Level was increased from Level 3 to Level 4 today — the second highest level — following an increase in Omicron cases.
The country’s four chief medical officers and NHS England’s national medical director had recommended the move to ministers.
As many as 1,239 confirmed cases of the Omicron mutation were recorded in the UK as of Sunday.
It brings the total number of UK cases of Omicron to 3,137 — a 65 per cent increase from Saturday’s total of 1,898 UK cases.
The new Level 4 grading means the epidemic is 'in general circulation, transmission is high and direct Covid-19 pressure on healthcare services is widespread and substantial or rising', according to the UK Government.