Boris Johnson has promised action to tackle online "disinformation" about vaccines that could prevent take-up.
Labour leader Sir Keir Starmer called for emergency legislation to fine social media firms who failed to clamp down on incorrect information.
Mr Johnson agreed online "anti-vaxxers" were a problem and promised a plan to deal with them "very shortly".
It cams as the UK approved the Pfizer/BioNTech coronavirus vaccine, paving the way for mass vaccinations.
The medicines regulator, the MHRA, says the jab is safe to be rolled out - and the first doses are already on their way to the UK, with 800,000 due in the coming days, Pfizer said.
England's Deputy Chief Medical Officer Jonathan Van Tam hammered home the message that people must take the vaccine at a Downing Street press conference with Mr Johnson.
"It is not going to help you if you don't take it," said Mr Van Tam, adding that "watching others take it" would not necessarily work when it came to suppressing the virus.
"You have to take the vaccine when it is offered to you. Low uptake will almost certainly mean restrictions will last longer," he added.
Research published in September suggested a fifth of the UK population would be likely to refuse a Covid vaccine.
The survey of 70,000 people by University College London suggested "substantial levels of misinformation amongst the general public about vaccines", with nearly 30% believing they can cause future health problems.
News that the UK has become the first country to license a Covid vaccine, and that the first doses will be arriving in the country next week, has been welcomed across the political spectrum.
At Prime Minister's Questions, Mr Johnson strongly urged people to get the jab, when it was offered to them, but stressed that it will not be mandatory, adding that compulsion was "not part of our culture or ambition".
Sir Keir Starmer said public confidence in vaccines was "going to be crucial to the success of getting this rolled out across the country, getting our economy back up and running".
He added: "We've got the highest regulatory and medical safety standards in the world but it's really important we do everything possible to counter dangerous, frankly life-threatening disinformation about vaccines.
"We on this side have called for legislation to be introduced to clamp down on this, with financial penalties for companies that fail to act, so will the PM work with us on this and bring forward emergency legislation in the coming days which, I think, the whole House would support?"
Mr Johnson said: "We are, of course, working to tackle all kinds of disinformation across the internet and he's right to single out the anti-vaxxers and those who I think are totally wrong in their approach, and he's right to encourage take-up of vaccines across the country, and we'll be publishing a paper very shortly on online harms designed to tackle the very disinformation that he speaks of."
Following PMQs, Labour's shadow foreign secretary Lisa Nandy tweeted criticism of Mr Johnson's answers.
The PM needs to be far more proactive about the dangerous disinformation spread by the anti-vaxxer movement. This rubbish puts us all at risk. Disappointing to see him give such a weak response to important questions from @Keir_Starmer at #PMQs— Lisa Nandy (@lisanandy) December 2, 2020
New legislation to combat "online harms" - including misinformation about vaccines - has been repeatedly delayed, since a White Paper was published last April, with ministers blaming the pandemic for the hold-up.
It is now expected next year.
The World Health Organization (WHO) says the world is not only fighting what it calls an "infodemic" - where an overload of information, some of it false, makes it difficult for people to make decisions about their health.
It is trying to answer people's concerns about the vaccines - as well as helping people evaluate the information they see on social media.
Facebook - which owns Instagram and WhatsApp - says it removes information that could lead to "immediate harm", including false claims about cures for Covid-19.
It also says it has banned ads that discourage people from getting vaccines, and reduced the number of people who see vaccine hoaxes.