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.