Booster jabs will be offered to everyone over 18 in England from this week, the PM has announced, as he declared an "Omicron emergency".

"No one should be in any doubt, there is a tidal wave of Omicron coming," Boris Johnson said on Sunday.

A new target has been set to offer boosters to all adults who want one by the end of the month, he said.

Ten people in England are in hospital with Omicron, Health Secretary Sajid Javid confirmed on Monday.

Some medical appointments are to be postponed to focus on boosters.

Mr Johnson gave his update in a TV statement on Sunday evening, just hours after the UK's Covid alert level was raised to four due to the spread of the new Omicron virus variant.

Level four means a high or rising level of transmission - the last time the UK was at this level was in May.

People have been told to work from home from Monday "if they can", as part of the UK government's introduction of Plan B measures.

"I'm afraid we're now facing an emergency in our battle with the new variant Omicron," said Mr Johnson.

"It is now clear that two doses of vaccine are simply not enough to give the level of protection we all need. But the good news is that our scientists are confident that with a third dose, a booster dose, we can all bring our level of protection back up."

He added: "At this point our scientists cannot say that Omicron is less severe.

"And even if that proved to be true, we already know it is so much more transmissible that a wave of Omicron through a population that was not boosted would risk a level of hospitalisation that could overwhelm our NHS and lead sadly to very many deaths."

The new booster target means people aged 18 and over in England will be able to get their third jabs from this week - as long as it has been three months since their second dose.

People aged 30 and over can already book an appointment in advance using the online service from two months after their second dose and over-18s in this position will be able to book from Wednesday.

Some walk-in appointments will be available from Monday for eligible over-18s, depending on location.

Scotland is also setting the same target and aiming to offer all adults a booster by the end of the year, and Northern Ireland said it is stepping up its rollout and hopes to get as many people as possible boosted by then.

But Mr Johnson said in order to reach the new jab target, certain other medical appointments would need to be postponed to the New Year. Some GPs are already allowed to postpone routine health checks to make space for vaccinations.

The prime minister also said:

Health Secretary Sajid Javid said on Monday that 10 people in England were currently in hospital with Omicron, with no deaths confirmed.

However, he told BBC Breakfast that 40% of all Covid cases in London were of the variant, which was spreading at a "phenomenal rate".

Asked whether the new measures were excessive, Mr Javid said it was important to act now, given the lag in hospitalisations in deaths that follow a rise in cases - even if Omicron turns out to be milder than other variants.

"We have seen what Covid is capable of... you start seeing a rise in cases, people get ill, some enter hospital, some sadly die," he said. "It's better to act early."

Early data suggests that getting a third booster dose gives an individual around 70% to 75% protection against symptomatic infection from Omicron.

More than half a million booster jabs and third doses were given in the UK on Saturday - the second day that has happened since the booster rollout began.

For people who are clinically vulnerable to Covid, a third dose of a vaccine is considered their full course - with a fourth jab being given as their booster.

Boris Johnson's stern words on Sunday night may run into three different challenges.

It won't be easy to expand the booster programme at such a pace.

There's been plenty of anecdotal evidence about the availability and eligibility, and questions about why it didn't get going much more quickly, weeks ago.

Second, Boris Johnson's credibility has taken a significant knock in recent weeks. Will the public, this time, be as willing to listen to him?

And in his own party there is frustration at his decision making and scepticism about what's going on.

The prime minister can make bold and urgent promises about the booster, but keeping them is something else.

Labour leader Sir Keir Starmer reiterated Mr Johnson's call to "get boosted" - adding: "So much has been asked of the British people, and time and again you have risen to the challenge. So let's keep our foot on the pedal and get Britain boosted to protect our families, friends and NHS."

Liberal Democrat leader Sir Ed Davey said it was in the national interest for everyone to get a booster jab, but criticised the government's rollout for being "far too slow", adding: "And now they're having to rush it in an emergency and cause all these pressures on NHS staff."

NHS Providers, which represents NHS hospitals, mental health, community and ambulance services, said the booster campaign offered an opportunity to contain the impact of Omicron but added the NHS was already "beyond full stretch" and that there would be an impact on other care as more staff became involved in the vaccination campaign.

And the Royal College of Nursing also welcomed the expansion of the booster programme but said nurses were "already facing huge demands under existing unsustainable pressures in every part of the UK health and care system". It called for ministers to take "every step needed" to slow the spread.

Scotland and Northern Ireland have already opened up boosters to over-30s - and those aged 18 to 29 in Scotland can get their booster jabs from later in the week.

First Minister Nicola Sturgeon said Scotland had the highest rate of booster jabs of all the UK nations - but warned it was possible that further measures might be needed in the weeks ahead.

The end-of-December promise is about enabling everyone aged over 18 who is eligible to have been able to have booked their jab.

It does not mean those jabs will be in arms.

But for it to have any impact on blunting the coming Omicron wave there needs to be a significant acceleration of the booster programme.

GPs are going to be crucial to increase the speed of vaccinations. It is them in particular who will be asked to prioritise jabs ahead of routine work for the next few weeks.

The promise applies to England, but the devolved nations are expected to follow suit. There have been 23 million boosters given across the UK - with another 23 million eligible for one.

At current rates of vaccination only a third of those will have got their booster by the end of the year. The pace needs to change - and quickly.

It is the latest rule change as part of the new Plan B measures - which also include Covid passports for some crowded events from Wednesday and the updated work from home advice. Both Wales and Scotland have said new rules could come in the next few weeks.

Mr Johnson's statement comes as the row continues over gatherings at Downing Street during last year's coronavirus lockdown.