A former Labour MP has branded the party's biggest donor "amateur, hubristic and irresponsible" in the row over union influence.
Eric Joyce said the outcome of the growing dispute in the party would determine its chances in the next General Election.
Joyce is standing down from the Falkirk seat after self-confessed "atrocious" behaviour in a bar brawl last year.
The battle to find a Labour candidate to replace him has caused a huge row, with the Unite union accused of signing up Labour members to make sure its candidate is successful.
In an article for The Guardian, Joyce attacked the "amateur, hubristic and irresponsible actions of a small number of Unite officials at the top of the organisation" and said party rules would require further reform.
The referral of the case to the police had "raised the stakes to the highest level".
In the past unions had been a "stabilising influence" on the party in the town, he suggested, using their "putative power" sensibly in recruitment and selection.
But this time there appeared to be a deliberate effort to "swamp" the party with new members - and "hysterical" libel threats when the MP wrote about them online, he claimed.
"Without Ed Miliband's decisive actions in commissioning the report, Unite would have pushed legitimate Labour members out of the way to install what would have been a Unite-first, Labour-second candidate and, quite likely, MP."
Joyce last week became embroiled in a bizarre spat on Twitter with a Labour MP over the issue, branding East Kilbride MP Michael McCann a "dimwit".
The dispute reached a new level on Friday when Labour announced it was referring alleged ballot-rigging to the police.
Party officials said they had contacted the Procurator Fiscal's Office in Scotland over claims Unite sought to pack the constituency party in Falkirk with its members to secure the selection of its favoured candidate for the safe Labour seat.
The move was angrily denounced by Unite general secretary Len McCluskey who dismissed the allegations as "nonsense" and accused the Labour leader of deliberately seeking a "punch-up" with the unions.