In a letter to the School Teachers' Review Body, the Education Secretary said that she "was not persuaded" by the case for capping the number of hours teachers work each week.
It has been seen as a victory for headteachers who have argued that improvements in teachers' workloads should not be at the expense of management flexibility.
However, Ms Morris also cast doubt on whether an annual limit on hours - heads' preferred option - would be workable. Monitoring hours could create increased bureaucracy, she said.
Ms Morris says that reducing workload will be her top priority over the next three years but says that any specific measures will have to wait for the outcome of the Government's review of spending in June. The review body is due to report at the end of April.
David Hart, general secretary of the National Association of Headteachers, described the letter as "long on aspirations and short on solutions". He said that the Government appeared to be rejecting limits on anything worth talking about and that its approach would not be acceptable to teachers.