Teachers survey suggests 1 in 3 want to quit
One in three teachers quizzed in a national survey have said they are thinking of quitting.
The National Education Workforce Survey suggested that more than 88% of teachers asked said they did not think they could handle their workload in agreed hours.
Plaid Cymru said it was a "damning reflection" on the Welsh Government; the Conservatives called it "alarming".
But the government said only 14% of the workforce took part.
The workforce survey spoke to 10,408 individuals. A total of 5,115 of those were school teachers.
The survey suggested that 33.6% of school teachers indicated that they intended to leave their profession in the next three years.
More than a third of teachers - 38.6% - said they were either not very or not at all familiar with the content and recommendations in Professor Graham Donaldson's report Successful Futures, which laid out proposals for changes to Wales' curriculum.
Full-time school teachers said they regularly work an average of 50.7 hours during an average working week.
Workload was shared as the least rewarding aspect of 78.1% of school teacher's roles.
The ability to manage workload within agreed working hours was a key issue for 88.3% of teachers, who either disagreed or strongly disagreed it was achievable.
Plaid education spokesman Llyr Gruffydd said: "This report is a damning reflection of the Labour Government's failure to make teaching an attractive profession for people in Wales.
"Not only are the figures alarming in terms of the number of those who note they cannot cope with workload demands - a staggering 88% - the survey also shows the potentially damaging impact of this failure to protect staff welfare on the education sector as a whole."
Welsh Conservative Shadow Education Secretary Darren Millar AM said: "These findings paint a picture of a teaching profession that's overworked and disillusioned about the future.
"That a third of Welsh school teachers plan to quit within the next three years, signals a looming staffing crisis of unprecedented proportions."
However, the government pointed to findings that 47% of teachers wanted to continue to develop their practice.
Ms Williams said in a statement to AMs that she was disappointed that only 14% of a workforce totalling 72,497 took part.
"There isn't a single answer to the diverse range of issues raised and there are no easy answers," she said.
"There are clearly a number of positive findings, such as access to professional learning and confidence in the delivery/use of ICT.
"However, we as a government must also acknowledge that there is more to do in ensuring our workforce feel they have the best possible support available.
"As a priority, I remain determined to tackle the long running issue of workload and ensuring teachers have the space and time to teach to the best of their ability, reducing unnecessary bureaucracy and ensuring that they are supported by excellent higher level teaching assistants and learning support workers."
As well as school teachers, of the respondents, 961 were further education teachers, 257 were further education support workers, 1,306 were school supply teachers, and 2,179 were school learning support workers.