The vaccine protects against mumps, measles, and rubella
Fewer than half of all five-year-olds in London have had the MMR vaccine and booster jab.
Figures from the NHS show only 49% of children had received the first and second dose of the vaccine by their fifth birthday.
The MMR vaccine protects against measles, mumps and rubella.
Last month the government began a campaign to raise MMR vaccination rates in England, amid growing concerns about a measles epidemic.
The MMR jab is first given at about 13 months of age and the child is then given a pre-school booster.
The Primary Care Trusts (PCT) with the highest percentage of children who had received the MMR vaccination and booster by their fifth birthday are Kingston (76%), Hammersmith & Fulham (69%) and Sutton & Merton (68%).
The PCTs that fare the worst are Newham (42%), Islington (43%), and Brent Teaching (43%).
James Cleverly, chairman of the London Assembly Health and Public Services Committee, said: "The gap between coverage levels achieved in the capital and the national average is widening.
"Compared to the country's average of 74%, only 49% of children in the capital have had the MMR vaccine and booster by their fifth birthday.
"This is unacceptable."
Experts said the MMR vaccine should not be a cause for concern, but vaccination rates fell following controversy about its safety.
A spokesman for NHS London said in August a grant of £60,000 was announced for each London PCT to to raise the uptake of the MMR vaccine.
"Many people consider measles to be a relatively minor childhood illness, but it can have serious complications," he said.
"Measles represents the single biggest avoidable infectious disease risk in London today."