Measles has returned to four European nations previously seen as free of the illness, according to the World Health Organization (WHO).
The disease is no longer considered eradicated in Albania, the Czech Republic, Greece and the UK.
"We are backsliding, we are on the wrong track," said Kate O'Brien of the WHO's Immunization Department.
Measles is a highly contagious and potentially fatal illness that causes coughing, rashes and fever.
The disease can be prevented through two doses of the MMR vaccine, which is available for free for all young children in the UK.
Countries are declared measles-free when there is no endemic transmission for 12 months in a specific geographic area.
Ms O'Brien said all four European nations that have lost their eradication status have "extremely high" vaccination coverage.
"This is the alarm bell that is ringing around the world: being able to achieve high national coverage is not enough, it has to be achieved in every community, and every family for every child," she said.
Health experts warn that lies about the measles vaccine have allowed the illness to spread in certain areas or communities.
What are the numbers?
All regions of the world showed an increase in measles bar the Americas, which saw a minor decline - although the US registered its highest number of cases in 25 years.
Close to 365,000 cases have been reported worldwide this year, the WHO said, almost three times as many as in the first half of 2018.
Dr O'Brien blamed misinformation about vaccines and called on social media companies and community leaders to provide "accurate, valid, scientifically credible information".
The Democratic Republic of Congo, Madagascar and Ukraine are suffering the largest outbreaks of measles.
Numbers of measles cases were steadily declining worldwide until 2016, when the illness began a resurgence.
WHO officials said the reasons for the renewed spread vary from country to country. Some people lack access to vaccination programmes, while others are misinformed about the illness and how to prevent it.