The GMB said that it had asked Labour to cut its ties with Showsec, the last bidder for the contract.
Labour's National Executive Committee already voted to boycott G4S last year, after it had handled security for 20 years.
It comes after Labour party general secretary Iain McNicol has been cautioned that the Home Office could cancel the conference if there was not adequate security in place.
GMB’s security industry national officer, Roger Jenkins, said the union would find it unacceptable to hire Showsec, after talks about union recognition broke down this week.
Roger Jenkins, GMB national efficer for the security industry said: “This is by no means sorted. We have repeatedly asked Showsec to agree to sign up to a standard recognition agreement that recognises GMB to negotiate pay and conditions, seeing its workers treated with the dignity and respect they deserve. The clock is ticking fast ahead of next month’s conference and the company’s continued refusal to recognise GMB is intolerable."
"Those employed in the private security industry must be allowed the choice to be represented by GMB - an independent, professional and forward-looking trade union working to better terms and conditions. If Showsec continues to show such disregard for unions then it has proven itself totally unsuited to the task of providing security for this event - and the Labour party leadership must immediately put an alternative in place to avoid the conference being scuppered.”
The Labour party conference, if it is to go ahead, is set to be one of the most terse in modern times, given the fractures between different wings of the party.
A spokesperson said that the party does not comment on commercial issues relating to the conference.