He will contest the seat on September 19 when the by-election is held to replace local Liberal member Don Randall, who died in July.

The south-east Perth seat has been held by a member of the Liberal party since 2001. Mr Randall held the seat with a margin of almost 12 per cent.

Mr Keogh is running against Liberal candidate Andrew Hastie, who is a former member of the SASR (Special Air Service Regiment).

Mr Hastie has been the focus of recent media reports connecting him to an incident in Afghanistan where soldiers amputated the hands of Taliban fighters in order to identify them. He was the commanding officer of the troop involved.

Mr Keogh is 33 years old and grew up in the Canning electorate. He has worked as a federal prosecutor and commercial litigation lawyer. He was also the youngest president in the history of the Law Society.

He put his name forward for pre-selection in 2013 but later withdrew it in the interest of party unity.

Law Society Senior Vice-President Elizabeth Needham said: "On behalf of the Executive, Council and all members of the Law Society, I thank Mr Keogh for his exceptional service. I wish him the very best in his future endeavours."

Ms Needham will now assume the role of acting president of the Law Society.

To focus on the issues important to Canning locals in the by-election, I have resigned as @LawSocietyWA President http://t.co/HmLRz6YXPz

— Matt Keogh (@mattkeogh) August 21, 2015