Boris Johnson is continuing to resist calls from health leaders for tighter Covid restrictions despite the rising levels of infections.
The British Prime Minister acknowledged the numbers were "high" but said they were "within the parameters" forecast by scientists advising the Government.
It came as the daily number of cases reported in the UK surpassed 50,000 for the first time since mid-July.
Speaking during a visit to a primary school in Co Antrim, Northern Ireland, he said the most effective way of combating the disease was to press ahead with the booster jab programme.
His comments follow calls from the NHS Confederation and the British Medical Association (BMA) for ministers to activate their winter Plan B for England amid fears the health service could be overwhelmed.
BMA council chair Dr Chaand Nagpaul said the refusal to introduce supplementary measures - including Covid passports, mask-wearing in crowded public spaces and a return to working from home - amounted to "wilful negligence".
However Mr Johnson said the successful rollout of the vaccination programme meant the country was in an "incomparably better" position than it was 12 months ago and that the Government was sticking with its Plan A.
"We are continuing with the plan we set out in July. We are watching the numbers very carefully every day," he said in a pooled clip for broadcasters.
"The numbers of infections are high but we are within the parameters of what the predictions were, what Spi-M (modelling group) and the others said we would be at this stage given the steps we are taking.
"We are sticking with our plan."
The Prime Minister urged the over 50s to come forward and get their booster jabs as soon as they become eligible.
"You get the call, get the jab. We have done about 4 million booster jabs already but as soon as you become eligible, as soon as you get that call, everybody over 50 should be getting that jab," he said.
"Ninety percent of the adult population has antibodies right now but we must fortify ourselves further.
"The numbers (of infections) are high, we can see what's happening, we can see the increase, now is the time to get those booster jabs."
His call came as the latest official figures showed an estimated 4.7 million booster doses have been delivered in the UK, up from Wednesday's total of 4.4 million.
NHS England said a record number of people had booked their booster through the national booking service on Wednesday, with 234,000 bookings.
At the same time were a further 52,009 laboratory-confirmed cases of the virus as of 9am Thursday, the highest daily number since mid July.
It follows a warning from Health Secretary Sajid Javid at a Downing Street news conference on Wednesday that new cases could reach 100,000 a day.
He said the Government could be forced to introduce new restrictions in the run-up to Christmas unless people took up their booster jabs and followed continuing guidelines on socials distancing.
One leading virologist, Dr Chris Smith of Cambridge University, said the numbers were already probably close to 100,000, with around half of all cases being asymptomatic.
"We are looking very hard, we're doing more than a million tests a day now, but probably we do have really high levels bordering on that sort of number at the moment," he told BBC Breakfast.
Meanwhile Labour leader Sir Keir Starmer said the vaccination programme was stalling and urged the Government to commit to a target of 500,000 jabs a day.
"The Government said that the vaccine would be the security wall against the virus. And now the Government is letting that wall crumble," he said.
"The booster programme has slowed down so much that at this rate, we're not going to complete it until spring of next year. The Government needs to change, it needs to get a grip."
Despite the rising case numbers, Downing Street insisted there was still spare capacity in the NHS and that Plan B would only be activated if it came under "significant pressure".
The Prime Minister's official spokesman said there were currently 95,000 hospital beds in the NHS, of which 7,000 were occupied by Covid patients, while around 6,000 were free.
"We will be very carefully monitoring hospitalisation rates, admission rates, ICU (intensive care unit) rates, the usual metrics we have used through this pandemic to judge how our NHS is coping," the spokesman said.
"Obviously we won't hesitate to act if needed."
The spokesman denied reports that officials were considering a Plan C to ban household mixing if the situation continued to deteriorate.
"It's important to stress that neither ministers nor officials are working on these proposals.
"That is not accurate," the spokesman said.
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