Hospitals in the south of England have seen a "real rise in pressure" as a new strain of Covid-19 continues to spread, it has been reported.
This comes after the whole of Sussex was plunged into Tier 4 restrictions to slow the spread of the new variant.
On Boxing Day, the South East Coast Ambulance Service said it experienced "unprecedented levels of demand".
The ambulance trust, which covers Sussex, asked the public only to call 999 after considering "all other options".
A tweet from the service read: "We are taking longer than we would like to get some of our calls.
"Please be assured that we will get to you as soon as we can so only call back if your condition worsens.
"Our staff are working so hard to get to patients as quickly as they can. Thanks for your continued support."
READ MORE >> Coronavirus: Brighton streets empty on first day of Tier 4
The new mutant coronavirus strain led to London and parts of southern and eastern England being rushed into the new Tier 4, effectively cancelling Christmas plans and imposing measures similar to previous national lockdowns.
London Ambulance Service described Saturday as one of the busiest days in history, as the number of coronavirus cases continues to rise.
While the president of the Royal College of Emergency Medicine said she saw "wall to wall Covid" at one London hospital and encouraged people to only call 999 for serious emergencies.
Dr Katherine Henderson told BBC Breakfast that there was a "great deal of difficulty" getting patients into wards on Christmas Day.
She said: "The chances are that we will cope, but we cope at a cost - the cost is not doing what we had hoped, which is being able to keep non-Covid activities going.
"It is always challenging in winter, nobody would say that it wasn't, but at the moment the level of patient need is incredibly high."
The new Tier 4 rules mean non-essential shops, hairdressers and leisure and entertainment venues are closed.
Households are also not allowed to mix but one person is allowed to meet with one other person outside in a public space.
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