Schools and colleges across Wales will remain closed until the February half term unless there is a significant reduction in coronavirus cases, the Welsh Government has said.
It has also been announced that the level four lockdown restrictions introduced at midnight on December 19 will remain in place.
Ministers have been carrying out a three-weekly review of the current restrictions. The next update will be on January 29.
Education Minister Kirsty Williams said the decision to keep schools closed comes as the pandemic reaches a ''significant point'' in Wales.
She said: ''We are taking this action today because this new variant [of Covid-19] is far more infectious and is leading to increased numbers of people being hospitalised.
''Schools and colleges have been safe and secure environments throughout the pandemic and that continues to be the case.
''However, the latest TAC (Technical Advisory Cell) report is clear on education’s contribution. We know that school and college settings can contribute to wider social mixing outside the education environment and, right now we all have to play our part in suppressing this virus.
''I know these times are hard but please remember that this pandemic will end.
''Our wonderful schools and colleges will soon be full with the sounds of happy children as they always were before this cruel virus took hold.
''I cannot wait for that moment and I am sure all of you feel the same.
''But until then, it is more important than ever that we continue to follow the rules and stay at home.''
Following the announcement, Qualifications Wales are cancelling spring assessments for GCSE, AS and A-Levels.
The planned assessments were due to take place between 22 February and 23 April but new arrangements will now be made.
Philip Blaker, chief executive of Qualifications Wales, said: “Following the decision that most pupils in schools and colleges will continue to learn remotely, the planned spring internal assessment window will not take place for GCSE, AS and A-levels and revised arrangements, adapting the plans previously announced, will be made.
"We know that learners need clarity about how they will be assessed and there will be clarity on revised arrangements soon.
“The Minister’s Design and Delivery Advisory Group will now be considering the way forward and more detailed information about revised arrangements will be made available shortly.’’
Leading education unions have welcomed the news that schools will remain closed for longer and said there was ''no other option at this moment.''
NAHT Cymru director Laura Doel said: “The health and safety of pupils and staff must be the priority and now we must turn our attention to supporting those learners at home and working with the government to reopen schools when it is safe to do so.
“Talks will begin today between trade unions and the government to establish and agree new Covid-related safety measures in schools during the restriction.''
Children's Commissioner Professor Sally Holland said closing schools for longer will have a ''significant impact'' on children.
She said: ''No-one wants to be in this situation and children and their families will be saddened and disappointed by the continued closure of schools and colleges for the majority of pupils.
''The focus currently has to be on doing all we can to minimise the inevitable impact on children, whilst protecting public health.''
First Minister Mark Drakeford today said the NHS was under immense pressure and that the new strain of Covid-19 is adding a "dangerous dimension" to the pandemic.
He said: "The coronavirus pandemic has reached a significant point.
"Cases in Wales remain very high and our NHS is under real and sustained pressure.
"The alert level four restrictions we introduced before Christmas must remain in place to keep us all safe. To slow the spread of the virus, we must all stay at home to protect the NHS and save lives.
"These feel like dark times but the new Covid-19 vaccines are being rolled out across Wales, giving us a path out of this pandemic.
"It will take a huge effort to vaccinate everyone and, despite the end of this pandemic being in sight, it is more important than ever that we follow the rules and stay at home.
"We have made so many sacrifices together and we mustn’t stop now.”
The government also said that the new Covid-19 variant - which is up to 70% more transmissible - had a 'firm foothold' in North Wales.
The First Minister said: "This new strain adds a new and unwelcome dimension to the pandemic.
"Wherever there's mixing; wherever people come together, the new strain is spreading. It is highly transmissible and spreads very quickly from person to person.
"We must all stay at home, protect the NHS and save lives. Together, we will keep Wales safe."
Mr Drakeford said he plans to strengthen lockdown measures for supermarkets and retailers, workplaces and schools and colleges.
"I want to see at least the same level of protection today as was very visible in the early period of the pandemic," he added.