It comes amid evidence that increasing pupil rolls, teacher shortages, and a lack of space are driving up class sizes in many schools.
Teachers debated the issue at the annual conference of the NASUWT, which took place in Birmingham over the Easter break.
An NASUWT survey published to coincide with the debate found that of around 3,000 teachers, 95% say class sizes are having a negative impact on their ability to meet the needs of all pupils.
A majority of the respondents also said that increasing class sizes are having a negative impact on pupil progress, attainment, and behaviour.
Teachers in the survey (across primary and secondary phases) reported an average class size of around 28 pupils. However, more than 75% said their class sizes have increased in the last five years.
When asked why the size of their classes have increased, 67% of the teachers cited an increase in the number of pupils on roll, 40% said it was due to cuts to staff numbers, and 40% cited budget cuts or financial pressures.
A motion passed by delegates called on the NASUWT to lobby UK governments and administrations to introduce a maximum limit on class size as well as to publish updated guidance to its members.
The motion stated: “Conference is concerned that class sizes are increasing in many schools, caused by a combination of increasing pupil rolls, teacher shortages and lack of space.
“Conference asserts that increasing class size has many detrimental impacts on teachers and pupils, including increasing workloads and decreased individual pupil contact.”
The motion also accuses some employers of ignoring the health and safety risks of overcrowded classrooms, such as when it comes to Covid transmission or in practical subjects like science.
Dr Patrick Roach, general secretary of the NASUWT, said: “Increases in class size numbers are having a detrimental impact on both the learning experiences of pupils and the health and safety of teachers and students.
“The damaging impact of increased pupil numbers in classes has been further exposed during the pandemic, creating the perfect conditions for the transmission of Covid-19.
“This situation once again exposes the failure of government oversight over the last decade in relation to pupil place planning or in guaranteeing the additional investment needed to increase teacher numbers.”