Around 5,000 children in the West Country are yet to be fully immunised against MMR.
Public Health England has released the figures as Prime Minister Boris Johnson visits the Royal Cornwall Hospital to announce plans to improve immunisation rates.
The agency is urging parents of children starting primary school in September to check their immunisation records.
According to PHE, some four and five-year-olds will be starting school next month at "unnecessary risk of serious diseases" because they aren't up-to-date with vaccinations.
In the UK dose 1 of the MMR vaccination protects against Measles, Mumps and Rubella and is usually given to children at around 12 months old.
The second dose is then given just before children start school to ensure the best protection.
Two doses of the MMR vaccine are needed for a person to be fully protected from the diseases.
PHE said it's "particularly concerned about children being at greater risk of measles" due to outbreaks across the region in the past year.
The warning from Public Health England comes as Boris Johnson is on his first visit to the West Country since becoming Prime Minister.
He's at the Royal Cornwall Hospital in Treliske announcing plans to improve immunisation rates after the UK has lost its measles-free status.
The new Prime Minister said "decisive action" was needed to improve rates and counter vaccine scepticism.