Two Liberal MPs will reportedly introduce a same same sex marriage bill next month with unprecedented support.

Sky News reports a cross-party bill, sponsored by Warren Entsch and Teresa Gambaro, will be brought to the Parliament on August 11.

Independent MPs Andrew Wilkie and Cathy McGowan will reportedly co-sponsor it, the network said, as will Labor MPs Terri Butler and Laurie Ferguson.

D-day for gay marriage: Two Liberal MPs will reportedly introduce a cross-party bill to introduce same sex marriage next month

Prime Minister Tony Abbott opposes same sex marriage. His sister, Christine Forster, is a leading advocate for same sex marriage

Sky political editor David Speers said the Liberal Party will decide on whether the party will be granted a conscience vote on August 18.

Prime Minister Tony Abbott opposes same sex marriage. His sister, Christine Forster, is a leading advocate for same sex marriage, and said today's development was 'tremendous news'.

After Opposition Leader Bill Shorten introduced a bill last month, Mr Abbott said a marriage equality decision would have to be owned by the whole Parliament.

'If our Parliament were to make a big decision on a matter such as this, it ought to be owned by the Parliament and not by any particular party,' he said.

Liberal MPs Teresa Gambaro (left) and Warren Entsch (right) - both from Queensland - will reportedly co-sponsor the bill

They will be joined by independent MP for Indi Cathy McGowan, reports say

Tasmanian MP and former intelligence officer Andrew Wilkie will also sponsor the bill - as well as two unnamed Labor MPs

Labor's Terri Butler and Laurie Ferguson are understood to have also thrown their support behind the bill

Advocates have said a bill would be unlikely to pass if Liberal MPs were not granted a conscience vote.

But parliamentary support has swelled in recent months - particularly following a string of huge victories for same sex marriage campaigners in the United States and Ireland.

The development was welcomed by Mr Shorten, who said in a statement he hoped Mr Abbott would allow Liberal MPs a free vote.

'As I've consistently said, it's the outcome that is important here, not whose name is on the Bill.

'Like millions of Australians, my first and only hope here is that we can make marriage equality a reality'.

Leading same sex marriage campaigner Alex Greenwich, the state NSW MP for Sydney, said: 'Here's hoping for spring weddings'.

But opponents were already organising on Wednesday afternoon.

Lyle Shelton, from the Australian Christian Lobby, tweeted: 'A bill to abolish husband & wife in the Marriage Act will be introduced to [Parliament] on Aug 11.

'Time to let MPs know kids need their mum & dad'.

The United States Supreme Court struck down that country's last remaining laws banning same sex marriage last week - the White House lit up in rainbow in celebration