After a marathon meeting lasting more than five hours, the Coalition party room has decided not to give MPs a free vote on same-sex marriage ahead of the cross-party bill to be introduced into parliament on Monday.

Two-thirds of the party room (66-33) spoke against allowing a conscience vote on the issue.

Of those, just three out of 21 Nationals voted for it, according to reports.

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A private members bill to legalise gay marriage was to be formally introduced to Parliament on Monday, with support from both Labor and Liberal MPs.

However, Tuesday night’s result means there will be no conscience vote in the Coalition party room and it is unlikely the bill will come to the floor of Parliament before the next election.

A Coalition MP said “the final vote (count) was 66-33 to keep the status quo. This was more like 75 per cent until the executive spoke up”.

“(It) proves the disconnect between some ministers and the grassroots and why we are having such problems.”

Earlier on Tuesday, Prime Minister Tony Abbott blindsided same-sex marriage supporters within the Liberal Party by suggesting the joint Coalition party room should decide whether to grant MPs a conscience vote on same-sex marriage.

The PM, who is an ardent opponent of gay marriage, made the play at the Liberals’ party room meeting on Tuesday morning.

He called a special meeting of the joint Coalition party room for 3.15pm, to discuss the issue.

Several hours later, MPs were still locked in talks.

“We’re having an open debate, a very forthright debate,” Environment Minister Greg Hunt told the ABC’s 7:30.

“[It’s] exactly what people would want from their parliamentarians.

“Every member, every single member who seeks to speak has the opportunity to speak.

“That’s something to be very proud of.”

Earlier, Education Minister Christopher Pyne warned the PM against the move, saying he could be accused of “branch stacking”, given so few Nationals MPs support gay marriage.

Same-sex marriage supporters within the party were shocked at the PM’s sudden announcement – which reportedly drew audible gasps from some of his MPs.

When asked about the party room discussions, Mr Abbott described the discussion as “brief and spirited”.

“I reviewed my pre-election statements and what I said was that this matter – same sex marriage – will be dealt with by the Coalition party room in the usual way,” he said.

“People in the party room were very much in the mind it should be dealt with as swiftly as possible.

“My position hasn’t changed, so let’s see what the party room comes up with later today.”

The opposition has allowed its MPs a conscience vote on gay marriage and the vast majority of its MPs have publicly declared their support for the legislation.

According to previous calculations by advocacy group Australian Marriage Equality, a slim majority of federal MPs support gay marriage, meaning the bill is likely to pass if Coalition MPs are allowed a free vote.

– with agencies