Mr Cox wrote to some MLCs about his concerns about the constitutionality of the proposed gay marriage bills before today's debate. 

His views are believed to have played a critical part in two members changing their minds to vote against the legislation, which appears likely to be defeated.  

In his letter, sent earlier this month, Mr Cox warns enacting legislation likely to be declared invalid by the High Court would be foolish. 

``Any such legislation would create a legal minefield in respect of rights and make Tasmania a legal laughing stock,'' he writes. 

Mr Cox, who has also spoken out against voluntary euthanasia, is also worried about eroding the status of marriage and the potential to create a ``stolen generation'' of children denied a mother and father. 

Yesterday, Gay and Lesbian Rights Group spokesman Rodney Croome issued a last-ditch plea for supporters to lobby five wavering upper house members. 

Mr Croome said any High Court challenge would cost about $50,000 compared to the millions gay couples would spend in the state on weddings. 

MLCs will be briefed by those on both sides of the issue before the debate in the upper house starts mid-afternoon. 

Premier Lara Giddings is not expected to address MLCs before the debate starts.

The Solicitor-General will provide an in-camera briefing to answer questions, but will not release his legal advice to the government.

With some members planning to speak for up to an hour, debate is expected to continue long into the night.