Single parents' charity Gingerbread says the reforms leave single parents high and dry.
Chief Executive Fiona Weir said: "The coalition government says it wants to make work pay and to support single parents into work, but the reality is that today thousands of single parents are being set up to fail.
"Government needs to act fast to improve availability of jobs with flexible working hours, improve the job-search support provided from day one of a claim, and increase access to childcare.
"Gingerbread welcomes the aim of new welfare reform proposals to make work pay, but the timescale for their implementation is too long for the single parents affected by today's change.
"We urge the government to introduce short-term measures to help make short-hours jobs pay, and to review recent decisions to cut help with childcare costs which will create a further barrier for single parents trying to get into work."