Australians have taken to social media to pressure Australian lawmakers to follow Britain's lead on same sex marriage.

British MPs have voted 400 to 175 in favour of allowing same sex marriage in a historic but controversial debate.

The issue was a trending topic on social media sites, with many Australian users supporting Britain's move and suggesting that Prime Minister Julia Gillard is now under pressure to change the law.

"Well done, UK, putting the Great back into Britain. Now for Australia to catch up," one Twitter user posted.

Australian Medical Association past president Professor Kerryn Phelps thanked British Prime Minister David Cameron for his leadership.

"It means so much to so many, even on the other side of the world," she added on Twitter.

"There is no justification for Australia continuing to refuse recognition of legal international same-sex marriages."

Melbourne-based Reuben Acciano added: "Don't be on the wrong side of history @Julia Gillard."

"Obama's flagged it as inevitable, now the UK. Listen."

And Sydney-based Steven Sommer posted: "Marriage equality has been passed in the UK with a conservative government in power. Why can't this happen in Australia with a Labor gvmnt?"

Britain is ruled by a coalition government made up of the right-leaning Conservative Party and socially progressive, left-leaning Liberal Democrats.

British MPs were allowed a free vote after Wednesday morning's debate and half of all Conservative politicians either voted against the proposed law or abstained.

The vote was taken in the country's lower house and must pass the House of Lords - equivalent of the Senate - before passing into law.

Critics of the proposed law appeared to be in the minority on social media sites - at least in Australia.

There were some in Britain.

"It is a step backward, an open display of rebellion towards God," British pastor Josh Williamson tweeted.