The maverick firefighters union is set to take legal action in an extraordinary bid to block moves to boost the number of female firefighters at Melbourne's key fire agency.

On Friday, the United Firefighters Union will seek to injunct the Metropolitan Fire Brigade's new selection process for recruits, including almost 300 female applicants, whose first test is scheduled for Sunday.

Nearly 300 women have applied to the MFB. Credit:The Age

For the first time, the MFB last month included quotas for the number of female and male applicants. And in a bid to increase diversity, those who pass written and physical tests will also have their gender and cultural background taken into account in the recruitment process, rather than being ranked on score alone.

Currently, slightly more than 3 per cent of MFB firefighters are female.