Put forward by the Association of Teachers and Lecturers (ATL) and National Union of Teachers (NUT), the motion said there was no evidence to justify selection in state education.
It comes as respected educationalist Andreas Schleicher, the head of education at the OECD, also warned that international evidence shows selection is not linked to school improvement.
The government is currently seeking responses to its consultation over the plans but has faced widespread criticism from the education community since prime minister Teresa May announced the idea.
The plan, which was not included in the Conservative 2015 General Election manifesto, is to “relax the restrictions on new or expanding selective schools” and allow existing non-selective schools to become selective “in the right circumstances”.
However, in launching the OECD’s annual education report last week, Mr Schleicher warned that selection tended to favour social background over true academic potential.
Notably, he said the evidence shows that when admission or selection is based on testing, “wealthy parents will find a way through it”.
He added: “What happens in most European systems is that academic selection becomes social selection. Schools are very good at selecting students by their social background, but they’re not very good at selecting students by their academic potential.”
Meanwhile, the TUC motion stated: “Congress notes that there is no evidence that grammar schools provide a route for poor, academically able children to achieve better life chances. Nearly all grammar schools have fewer than 10 per cent of pupils eligible for free school meals.
”The Institute for Fiscal Studies has found that deprived children are significantly less likely to go to a grammar school than their more advantaged peers, even when they do they achieve the same academic levels aged 11.”
The motion, which was passed, called on the TUC to work with the education unions to “oppose any increase in academic selection”.
Dr Mary Bousted, ATL general secretary, said: “The government has failed to provide any evidence that grammar schools are the solution to improve the education for the majority of children.
“Neither is there any evidence that grammar schools provide a route for poor, academically able children to achieve better life chances. Existing evidence does, however, show that pupil selection discriminates against the poorest children.
“This policy will not benefit the majority of children, but will benefit a chosen few, mainly from wealthier families. Evidence shows pupil selection lowers the attainment of the children who do not get into grammar schools.”
- The deadline for responses to the consultation is December 12 and the full consultation document is available via http://bit.ly/2cQWcgI