An internal brawl within the Australian Labor Party has reared its head before the party’s national conference, with lawyers for the Health Workers Union threatening to derail the event.

Lawyers from Phi Finney McDonald wrote to Labor’s national executive on Sunday alleging the union was being denied its right as “paid-up affiliate” for the convention.

Decisions at the event are voted on by delegates, with unions and branches allocated numbers of votes at the convention based on their size.

But in the fallout of the Adem Somyurek scandal and coronavirus shutdowns it was decided to Labor’s next national convention, to be held online, should using the same mix up as in 2018.

A frustrated HWU has demanded a greater number of delegates as its numbers have grown during the course of the pandemic.

The issue has also been partially linked to the dramatic intervention by the national executive into the Victorian branch triggered by the Somyurek saga.

In the legal letter, the lawyers state that the union was assured they would be awarded delegates based on ballots from 2020.

They then claim they were only informed of the decision to use 2018 figures on February 15 this year.

“We advise we have instructions from our client to commence legal action against the ALP if

it refuses to facilitate proper representation at the 2021 National Conference,” the letter reads.

“The National Executive decision to ignore the validly cast 2020 Union Ballots and instead

choose to rely upon the 2018 Delegates List to determine union delegates to the 2021

National Conference is invalid and beyond power.”

The right-aligned HWU last year called for then Health Minister Jenny Mikakos to be sacked  and the union has a rocky relationship with the Andrews Government.

The move comes as former Labor minister Marlene Kairouz gears up for a bruising Supreme Court battle to stop internal party processes which could see her expelled from the party.

Ms Kairouz was asked to front Labor’s internal disputes panel following the national intervention, which her legal team are expected to argue was invalid.