The UK faces a "tidal wave" of the Omicron variant of coronavirus and two vaccine doses will not be enough to contain it, UK Prime Minister Boris Johnson warned, as he accelerated the booster rollout programme.

Speaking hours after government scientists lifted the Covid alert level to 4 on a 5-point scale, Mr 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," Mr Johnson said in televised statement this evening.

"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.

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"Everyone eligible aged 18 and over in England will have the chance to get their booster before the New Year," he said.

The mission to administer millions of jabs by 31 December will see 42 military planning teams deployed across every health region, extra vaccine sites and mobile units, extended clinic opening hours to allow people to be jabbed around the clock and at weekends, and the training of thousands more volunteer vaccinators.

In his address, Mr Johnson said that the UK Government will provide additional support to accelerate vaccinations in Scotland, Wales and Northern Ireland.

Shortly after Mr Johnson's address, the UK's NHS website crashed as people rushed to book their 19 booster doses.

Last week, Mr 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 Mr Johnson's own Conservative Party lawmakers set to vote against these measures, the Prime Minister has said there are no plans for further restrictions as cases soar.

The seven-day average of Covid-19 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 today, the UK 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.

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In recent weeks, questions have once again emerged over Mr 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 British people said they were less likely to follow Covid-19 rules if government officials had broken them.