A £33m project to provide the poorest families with extra cash for childcare has been announced by the mayor.
Childcare can be 25% more expensive in London, says the LDA
Ken Livingstone said 3,255 nursery places would be subsidised to help parents who want to work.
Childcare costs up to 25% more in London than the rest of England and the London Development Agency said the money would provide "vital support".
But the subsidies will only be available to those parents on the highest child tax credit.
It is the first stage of a three-year programme, in which 10,000 places are expected to be subsidised.
The first places will be available from 14 November and the subsidy will keep full day care costs down to £175 a week - the child tax credit ceiling.
The project, funded by the London Development Agency and the government, also aims to prompt nurseries and accredited child minding networks to offer more flexible hours.
Mr Livingstone was at the Harmony Nursery, in Shepherd's Bush, west London on Wednesday - one of the first to pilot the project - where the poorest parents will be offered £30 subsidies.
It will only replace a previous grant which was cut earlier this year but more part time and flexible places will be provided, said BBC London's political editor Tim Donovan.
But Mr Livingstone said: "Here in London there's about 15%, mainly young women, who would like to go back to work but can't because they can't afford childcare.
"If we can get that 15% back into work, the impact on London's economy will be amazing."
But Liberal Democrats on the London Assembly said in many parts of London, demand for nursery places outstrips supply.
"The trouble is that the Childcare Affordability Programme doesn't create new places, but only makes existing childcare more affordable," said their childcare spokeswoman Dee Doocey.
"The mayor must stick to his promise of delivering the extra 2,350 new childcare places this year...otherwise the capital will still be short of what London's parents need."