Boris Johnson has slammed anti-vaxxers for spreading "mumbo-jumbo" on social media.
The Prime Minister said it was a "tragedy" that the NHS was under such pressure but people were "spouting complete nonsense about vaccination".
Mr Johnson said he would maintain a "voluntary approach" to jabs in the UK rather than making vaccines mandatory as in some other countries .
He told reporters on a visit to a vaccination centre in Northampton: "What a tragedy that we've got all this pressure on the NHS, all the difficulties that our doctors and nurses are experiencing and you've got people out there spouting complete nonsense about a vaccination.
"They are totally wrong and I think it's time that I, Government, call them out on what they're doing.
"It is absolutely wrong. It's totally counterproductive and the stuff they are putting out on social media is complete mumbo jumbo."
The PM admitted the NHS was facing huge pressure as the number of NHS trusts reporting critical incidents rose to 24.
But Mr Johnson insisted the health service had the staff to cope with the challenge posed by soaring cases of the Omicron variant.
He said: "First of all, yes, I appreciate that the NHS is under huge pressure and yes, you're quite right in what you say about the way it's been continuous over the last 18 months - we've had wave after wave of Covid and our NHS has responded magnificently and they've kept going.
"And of course I understand how frustrating it is to see another wave coming in, and I thank doctors, nurses, all health staff, everybody, for what they're doing to keep going."
But he said staff numbers had been increased, and that combined with the calling back of retired staff and volunteers would ease the strain.
He insisted Omicron was milder than previous variants but said: "The pressures on hospitals are clear.
"What we've got to do is give the NHS all the help we can through the next period, with all the simplifications of systems, moving staff from one hospital to another, all the ways we can back staff up, but also make sure that the people who are likely to get ill get vaccinated first.
"The saddest words in the English language are 'too late'. When you're in ICU and you haven't been vaccinated, sadly it's too late to get vaccinated, so get boosted now."
