Hillingdon borough has come under fire from a London Mayoral candidate.
Labour party's Tessa Jowell has criticised services that are "leaving children behind".
Analysis by the House of Commons Library shows that in 2014, 48% of children failed to achieve the expected level of development aged five.
The figures make Hillingdon the worst-performing borough in all of London.
Ms Jowell said: “We should all be shocked and galvanised to action by these figures which tell a story of nearly half of Hillingdon children beginning on a path that will take them to a lifetime of inequality and denied opportunity.
“What is even more shocking is that this waste of potential can be prevented by support for very young families and children in the first 1,000 days of their life to ensure that when they get to school they are ready for school and ready to learn."
Ms Jowell has previously announced a £61m a year fund to support development in a child’s first 1,000 days.
She added: “That’s why if I become Mayor of London I will act immediately to restore Sure Start to its founding purpose and ensure that every child has every chance.”
Cllr David Simmonds, Deputy Leader of Hillingdon Council and Cabinet member for education and children's services, spoke out when the figures were first released.
He said: “The council is holding schools and other providers rigorously to account for their performance and challenging Ofsted to play fulfil their responsibility to drive improvement.”
The five disciplines measured in the research are: communication and language, physical development, personal, social and emotional development, literacy and mathematics.
Hillingdon was seen to have the highest proportion of children failing to achieve the expected standard in communication, at 31%.
