Kenya National Union of Teachers (Knut) Secretary-General Wilson Sossion has asked members to boycott training on the new curriculum, that is scheduled to begin on April 23.
More than 100,000 teachers are set to benefit from a capacity-building training as the government takes steps to fully rollout the new system of education.
In a press statement on Wednesday, Mr Sossion explained that the competency based curriculum (CBC) reform exercise and the implementation process were conducted in violation of the law.
"The law requires an exercise of such a magnitude to have guidelines and a gazetted commission to anchor the entire process," he said.
"Besides, the due process of reforming the content of the curriculum was never followed to the letter, hence making the entire exercise illegal."
He also noted that the union's annual delegates' conference in December 2018 resolved that since the curriculum was not conducted in line with established norms, all teachers countrywide should boycott it.
The secretary-general also said that more importantly, a research conducted by Knut between January and March revealed the government was not fully prepared for the exercise.
He said there were no teaching and learning materials and also cited the teacher shortage and lack of standard infrastructure.
"We cannot admit an inappropriate and ill-prepared for learning system into our country," he said.
Amid the criticism, however, the Ministry of Education has insisted that the exercise is on course.
Between April 23 and 26, a total of 68,490 teachers in lower primary schools and 22,830 head teachers will be trained.
Master trainers will offer support during the training.
Early in April, headteachers of primary schools questioned the implementation of the new curriculum which started in January in pre-primary 1 to Grade 3.
Appearing before the National Assembly's Education Committee, Kenya Primary School Heads Association chairman Nicholas Gathemia poured cold water on the whole process - from piloting to implementation - citing several challenges.
Mr Gathemia told the committee that only two teachers were trained per school, and that it was inadequate as only two sessions were conducted and most of those trained were exiting the service.
"All teachers should be effectively trained. The training should be continuous and enhanced in teacher training colleges for the competency based curriculum to be incorporated in programmes," he proposed on behalf of the association which has about 20,000 members.
Headteachers also said the continuous assessment of individual pupils was a nightmare. They encouraged rot assessment in the case of classes with between 60 and 100 pupils.
Mr Gathemia also informed the committee of the need to build the capacity of teachers, improve the teacher to pupil ratio and build more classes in public schools.
On the piloting of the new curriculum which was done for two years, the chairman said there was bias in the selection of pilot schools. He said those chosen in each category - rural, urban and special needs education - had the best infrastructure.
"Our prayer is that the implementation of the new curriculum and other intervention is progressive for the benefit of all," he said.
Meanwhile, the Teachers Service Commission (TSC) says it needs Sh900 million to support the training of 100,000 teachers in April, August and December.
The teachers to be trained are those of Grade 1, 2 and 3.