The Victorian Labor Party has been rocked by allegations of widespread branch-stacking, involving thousands of its members, rigged preselection votes and illegal money-making activities.

Hardline Firefighters Union boss Peter Marshall has written to Labor authorities demanding an investigation into the recruitment of up to 4000 party members and into the conduct of party fundraising events that featured high-profile Labor figures as speakers.

Firefighters union boss Peter Marshall has taken an internal feud to the next level with explosive claims against his factional enemies. Credit:Angela Wylie

But the main target of Mr Marshall's accusations, former minister Adem Somyurek, strongly denies the claims of the union leader, saying Mr Marshall had "a tiny union, a small brain and a massive ego."

The party was not making any comment on Wednesday about Mr Marshall’s allegations, which dramatically escalate his feud with former emergency services minister Jane Garrett and threaten to plunge Labor into fresh internal turmoil with a state election four months away.