Labour's annual conference in Liverpool could be cancelled in a row over security, a trade union has claimed.
The GMB said the refusal by a security company to sign a union agreement was against Labour's rules.
It is calling on the party to find an alternative contractor for next month's event to avoid it being "scuppered".
The company, Showsec, said it was "not anti-unions in any way" and Labour sources said they were confident the conference would go ahead as planned.
But a Labour spokesman said: "We do not comment on commercial arrangements."
The BBC understands the party has not yet signed a contract with a security provider for the conference, which begins on 25 September.
But it has rejected newspaper claims the Home Office could shut down the four-day event, at Liverpool's Arena and Conference Centre, if Labour fails to sort out its own security, with Labour sources saying the conference is a "private event".
Labour approached Showsec after the party's National Executive Committee voted last year to boycott G4S, which has provided security at party political conferences in the UK for some years.
It was prompted by claims by the Palestine Solidarity Campaign that G4S was linked to "Israeli prisons which hold Palestinian political prisoners from occupied Palestinian territory inside Israel".
The UK's main party conferences are held inside secure zones, with airport-style security scanners to screen the thousands of attendees.
Three security companies are reported to have declined a contract with Labour before it invited Showsec to tender.
Roger Jenkins, GMB national officer 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."
He added: "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."
Mr Jenkins also told The Guardian Labour rules stipulated that "any service provider they use have to have a recognition agreement with a trade union".
In a statement, Showsec said: "We were invited to tender for the security provision at the Labour Party conference, but at no point were we advised that signing a recognition agreement with the GMB Union was a stipulation of that tender.
"Showsec are not in a position to sign this recognition agreement due to corporate governance guidance, but contrary to some reports, we would like to make it clear that the company are not anti-unions in any way.
"We have, in fact, had a partnership agreement in place with GMB Union since 2004, but following discussions with the union in an attempt to find a resolution, they have advised us that this is no longer fit-for-purpose.
"We are disappointed that we presently find ourselves in this position."