Division over the details of the same-sex marriage plebiscite continues within the government.

Opposition leader Bill Shorten has said the Prime Minister would be a “traitor to the cause” if he does not allow a debate on same-sex marriage during the new session of parliament on 18 April.

Ahead of a joint event on marriage equality hosted by the Guardian Australia and Australian Marriage Equality (AME), where the Labor leader will be speaking alongside Greens leader Richard Di Natale and AME’s National Director Rodney Croome, Shorten told the Guardian Australia Malcolm Turnbull has “run out of excuses."

“Malcolm Turnbull can use the recalled sessions of parliament for a free vote on marriage equality,” he said. “His delaying tactics aren’t fooling anyone. If he refuses to do so, it will prove once and for all that he is a traitor to the cause.”

Turnbull recalled the parliament for an 18 April sitting to debate the Australian Building and Construction Commission legislation, which many speculate will be used as a trigger for a double dissolution and early election that may take place on 2 July. However, the Senate, which does not have a Coalition majority, could choose to debate other legislation, including same-sex marriage.

Shorten did not promise that Labor would use its numbers in the Senate to drive the discussion towards marriage equality rather than the building watchdog legislation, but instead restated his election commitment on marriage equality.

“If Malcolm Turnbull is too weak to confront the right of his party, Labor will have the parliament vote within 100 days of being elected,” he said.

Shorten’s comments come just weeks after it was revealed the government has delayed plans to finalise the details of the same-sex marriage plebiscite.

The Turnbull government had previously committed to reveal the details of its planned marriage equality plebiscite before the election. With a double dissolution on the table, voters could go to the polls without knowing the full scope of exemptions to discrimination legislation, or details of public funding.

The Attorney General George Brandis had consulted with all sides of the debate and had been expected to bring a submission to cabinet in mid-March so the Coalition party room could be consulted and the details announced before the election was called.

But it is believed the issue has divided the Coalition, with conservative MPs demanding broad exemptions from anti-discrimination laws for officials and wedding service providers.

It is understood that the government has been considering “very limited” public funding for both sides of the campaign as well as providing an official government-produced brochure outlining each case. The cabinet must also decide the exact question to be used in the plebiscite.

However, if an early election is called, it is expected the next party room meeting will be during budget week, where it is unlikely the Coalition will want to encourage further division among its members before the campaign trail.