The cost of pre-school childcare is barring 50% of single parents in London from going to work, a charity has said.
A nursery place eats half the income of the average single parent, once housing is paid for, Gingerbread claimed.
Weekly costs (average of 25 hours) are at £158.73 in the capital, £47 more than the national average, Family and Childcare Trust figures indicated.
Some parents have no choice but to hand in their notice during summer holidays, Gingerbread said.
Fiona Weir, its chief executive, said affordable childcare was essential to enable single parents could get paid work.
"For parents of pre-school children, the situation is even more challenging, with little affordable support in place," she said.
"This, coupled with the tightening-up of eligibility criteria around the 30 hours of free childcare for parents of three and four-year-olds, means significant barriers need to be overcome."
Gingerbread wants the Greater London Authority to pay the deposits nurseries and childminders typically require, to help parents back into work when they have yet to receive a pay cheque.
The charity's analysis found 4,500 single parents in London would be ineligible for the government's 30 hours of free childcare.
And 24,000 single parents of preschool children would now have to seek work or risk a benefits sanction.
While increasing numbers of single parents were in work, over half of them were living in relative poverty, on average between 2011 and 2014.
'Want to go back full-time'
"I've scaled back my hours to 20 hours per week because I know what the financial implications of having to work 9-5 mean in terms of childcare," one single mother, from Southwark, told Gingerbread.
"I also know that come the six-week school summer holidays I'm going to have to give up my job because I will not be able to afford the childcare costs."
Another mother, from Enfield, told the charity: "If I want to go back to work full time I would I have to pay nursery costs but for me it's too expensive. It would cost me £1,580 per month, so I... will only be returning to work part-time.
"Even then I have had to borrow money from family, as it still costs me £700 per month."