Those asked to shield against Covid during the first wave of the pandemic in Wales have been advised to avoid leaving home for work or school.
About 130,000 people deemed extremely vulnerable due to underlying health conditions were originally advised to stay at home and isolate from others.
Now a rise in cases possibly linked to a new variant of the virus has prompted the Welsh Government to tell those affected not to attend work or school.
Health Minister Vaughan Gething wrote in a statement that the advice, which is effective immediately, was particularly relevant to those whose work required them to be in regular or sustained contact with people, or who shared a poorly ventilated workspace for long periods.
Mr Gething wrote: "This decision has been taken based on number of factors but has been influenced most recently by the significant recent growth in rates of infection, possibly due to the new variant of the coronavirus.
"We have also taken account of the pressures we see on our health service with increasing hospitalisations."
Letters will be sent from the chief medical officer (CMO) and can be used as evidence to claim statutory sick pay.
People shielding can still go out to exercise and attend medical appointments, and are able to remain part of a support bubble.
Mr Gething added: "We have been clear that the safest option for people within this group is not to be part of a Christmas bubble.
"However if they choose to do so they should follow the advice provided on our website which includes keeping contacts to an absolute minimum, meeting for short periods in well ventilated areas, maintaining strict hand and surface hygiene and staying two metres away from others."
He acknowledged that letters informing people of the change were likely to be delayed because of the Christmas period.
The Welsh Conservatives had urged the government to introduce "a compassionate shielding process", which would include food boxes, dedicated supermarket home delivery shopping slots and financial support for those who are unable to work.
The party's health spokesman, Andrew RT Davies, said: "I'm pleased that the Welsh Government has thought about the most at risk in our communities however, today's announcement does not go far enough.
"What this pandemic has shown is that half measures are not good enough, especially when there are reports today that Wales has the highest rate of Covid-19 infections per 100,000 people in the world."
Plaid Cymru Member of the Senedd for Rhondda Leanne Wood said she welcomed the decision.
"I have had people in Rhondda Cynon Taf tell me they were put in a terrible position of having to choose between risking their lives in work or staying at home and facing poverty because they had no right to stay at home," she said.