One of the last barriers to same sex marriage within the labour movement could be crumbling, as the powerful “Shoppies” union moves to drop its long-held and fierce opposition.

Guardian Australia understands the powerful and socially-conservative Shop, Distributive & Allied Employees’ Association (SDA) has discussed dropping its opposition to marriage equality at both its national executive and national conference in favour of having no stated policy on the issue.

The Victorian, South Australia, Western Australia and Newcastle branches are understood to back the change while the Queensland, New South Wales and Tasmania branches are reluctant to shift.

Sources say that puts the numbers “line ball” while others say those advocating change have “narrowly got the numbers”.

The possible shift at the largest Labor-affiliated union comes after generational change with Joe de Bruyn retiring as national secretary and WA senator Joe Bullock standing aside as WA secretary of the SDA when he entered parliament in 2014.

The union’s controversial views on marriage equality have long been suspected to be out of step with its membership where young, female and casual workers are heavily represented.

This is understood to have been confirmed by research done for several branches of the union which showed its members were in favour of gay marriage or held neutral views on the issue.

The shift, which will be discussed again by the union’s national executive within weeks, would change the power balance within Labor party policy making forums such as national and state conferences but it is unclear the extent to which it would change a parliamentary conscience vote.

Several SDA aligned members and senators have declared their support for marriage equality despite the view of the union. Other MPs and senators aligned with the union could still vote against marriage equality in a conscience vote.

Joe de Bruyn told Fairfax Media earlier this year “marriage started with Adam and Eve” and it was an “objective” truth same-sex couples could not marry.

Joe Bullock was forced to apologise for a speech which emerged as he was preparing to enter the Senate in which he called some Labor party members “mad” and also said:

The time is coming where faith needs to be protected from politics … the anti-discrimination movement is forcing religious belief into the four walls of a church. I am conscious of the movement for homosexual marriage and what that could mean for Catholic schools, where homosexuality could be required to be advocated as a moral equivalent to heterosexual marriage … this is politics invading freedom of religion … freedom of religions is the freedom to practice your religion and that means taking your beliefs out of your church and living them in your life.

It is estimated there are about 11 MPs with links to the SDA, including MPs Tony Burke, Kate Ellis, David Feeney, Amanda Rishworth and Nick Champion and senators Helen Polley, Jacinta Collins, Chris Ketter, Deborah O’Neill, Catryna Bilyk and Bullock.

The MPs have all declared their support for marriage equality. The senators are all opposed except for Queenslander Chris Ketter who has not declared a position.

There have been protests in support of same sex marriage by young retail employees outside the union’s offices, complaining the union’s position does not reflect their views.

At the Labor party conference in July a compromise deal decided Labor MPs and senators would maintain a conscience vote for two terms of parliament on the issue before they were bound to vote in favour of it.

The left had appeared to have the numbers to win a resolution binding Labor MPs to support same-sex marriage in the next term of government, contrary to party leader Bill Shorten’s position in favour of a conscience vote.

They agreed to the deal to push the binding vote out to the following term amid concerns from some MPs and from the SDA.

New prime minister Malcolm Turnbull promised to retain Tony Abbott’s policy of holding a plebiscite on marriage equality in the next term of parliament, but is under pressure to spell out the details of how that would occur.