Government sources said his decision was related to a police investigation into an alleged assault.

Mr Theophanous, responsible for major events, industry and trade and information technology, said late yesterday he would stand aside during the investigation.

He said the nature of the complaint was a mystery to him.

A police spokeswoman said last night it was not appropriate to identify anyone under investigation ''until such time as they have been charged''.

Mr Theophanous, a 20-year parliamentary veteran who has been in cabinet twice, said he was confident he had ''no case to answer''.

''Following a series of rumours over the last week or so, today I decided to contact the Victoria Police and ask them if a formal complaint which involved me has been lodged with them and whether they were inquiring into it,'' Mr Theophanous said.

''They have confirmed that this was the case.

''I feel strongly that if a minister is the subject of a formal complaint which is being investigated by the police he or she should stand down until the matter is cleared and cleared up by the police.

''Even though I know that I've done nothing wrong, the test for ministers in a government is rightly high,'' he said.

''I am aware of my responsibility to protect the Government and the work it is doing for Victorians.''

Mr Theophanous said he had contacted police and they indicated he might be interviewed soon.

''While I have not been advised by the police as to the nature of that complaint, I have been advised that the police are conducting interviews in relation to the matter,'' he said.

''I have been further advised that Victoria Police may wish to interview me in the near future in relation to their inquiries.''

Mr Theophanous will also seek leave from Parliament until the matter is cleared up.

Premier John Brumby accepted Mr Theophanous's decision when informed yesterday afternoon.