Wales will be placed under lockdown from midnight.
First Minister Mark Drakeford told people to "stay-at-home" as Wales goes straight into alert Level 4 from tonight - with non-essential retail and hospitality firms to close.
Plans to relax rules from 23 to 27 December to allow people to celebrate with loved-ones have been scrapped but meeting up is allowed on Christmas Day.
Wales had been due to go into Level 4 lockdown on December 28.
Mr Drakeford said hundreds of people had already contracted the new "more aggressive" strain of the virus in Wales.
Public Health Wales said the immediate introduction of new restrictions is related to the "identification of a new more transmissible variant of Coronavirus".
The Welsh Government held an emergency cabinet meeting earlier on Saturday and made the shock announcement this afternoon.
It comes as Prime Minister Boris Johnson has introduced lockdown measures from midnight for London and the south east England in response to rising cases.
Most Read
"While we all want to avoid further disruption to businesses and plans for Christmas, our overriding duty is to protect lives here in Wales," Mr Drakeford said.
Hospitality and retail businesses have been left devastated after buying in stock for the festive period.
Ben Francis, FSB Wales Policy Chair, said: “Clearly these are very difficult times for Wales and there is a significant amount of concern in all areas about the continuing rise of coronavirus.
“Closing down businesses now, when non-essential retail and hospitality firms are trying desperately to maximise whatever opportunities they can from the pre-Christmas period (ordinarily their most important time of year), is nothing short of devastating for thousands of businesses across Wales.
“Today’s announcement means that many businesses will be left with substantial stock that reflects a financial liability for them, and we will be looking to speak to Welsh Government about how businesses can be supported in the coming days."
Jim Jones, chief executive of North Wales Tourism, said he was "very disappointed" with the "knee jerk reaction without proper thought given to the huge damage you have just inflicted on the morale of the population of Wales".
Ian Price, CBI Wales Director, said: “With a more infectious strain of the virus in circulation it’s no surprise to see planned restrictions brought forward, however painful they may be.
Don’t miss
“It is for the Welsh Government and their health advisors to take the lead in setting rules for the festive period, and public wellbeing will rightly be at the forefront of their thinking. Businesses know infection rates must be controlled to save lives and protect against longer-term economic scarring.
“Nevertheless, invoking what is effectively a full lockdown at such a crucial time for many businesses – particularly those in the hardest-hit hospitality, retail and tourism sectors – will come as a fresh blow after many months of hardship and once again puts further jobs at risk.
“It’s vital that any change to rules – anywhere in the 海角视频 – is shaped by clear evidence, consistently applied, and accompanied by increased support for businesses in the worst-hit sectors. Vaccinations are now underway and offer tangible reasons for optimism in 2021 – the government must do everything possible to help businesses survive until risks recede and trade returns.”
Paul Davies, leader of the Welsh Conservatives, said he had been briefed by Wales’ First Minister Mark Drakeford on Saturday and appreciated the need for “more severe restrictions” in the country.
Leader of Plaid Cymru Adam Price MS said: “In light of the strongest possible scientific advice, it is right to take decisive action to bring the level of cases under control, save lives and ease the mounting pressure on the NHS."